Change comparison operators to allow nulls (#8617)

Prior to this PR, the less/greater than operators (`<`, `>`, `<=`, `>=`)
would throw an error if either side was null. After this PR, these
operators return null if either side (or both) is null.

### Examples
```bash 
1 < 3       # true
1 < null    # null
null < 3    # null
null < null # null
```

### Motivation

JT [asked the C#
folks](https://discord.com/channels/601130461678272522/615329862395101194/1086137515053957140)
and this is apparently the approach they would choose for comparison
operators if they could start from scratch.

This PR makes `where` more convenient to use on jagged/missing data. For
example, we can now filter on columns that may not be present in every
row:
```
> [{foo: 123} {}] | where foo? > 10
╭───┬─────╮
│ # │ foo │
├───┼─────┤
│ 0 │ 123 │
╰───┴─────╯
```

crates/nu-command/tests/commands/where_.rs

@@ -194,3 +194,10 @@ fn fail_on_non_iterator() {
 
     assert!(actual.err.contains("only_supports_this_input_type"));
 }
+
+// Test that filtering on columns that might be missing/null works
+#[test]
+fn where_gt_null() {
+    let actual = nu!("[{foo: 123} {}] | where foo? > 10 | to nuon");
+    assert_eq!(actual.out, "[[foo]; [123]]");
+}

crates/nu-parser/src/type_check.rs

@@ -295,6 +295,9 @@ pub fn math_result_type(
                 (Type::Custom(a), Type::Custom(b)) if a == b => (Type::Custom(a.to_string()), None),
                 (Type::Custom(a), _) => (Type::Custom(a.to_string()), None),
 
+                (Type::Nothing, _) => (Type::Nothing, None),
+                (_, Type::Nothing) => (Type::Nothing, None),
+
                 (Type::Any, _) => (Type::Bool, None),
                 (_, Type::Any) => (Type::Bool, None),
                 _ => {
@@ -322,6 +325,9 @@ pub fn math_result_type(
                 (Type::Custom(a), Type::Custom(b)) if a == b => (Type::Custom(a.to_string()), None),
                 (Type::Custom(a), _) => (Type::Custom(a.to_string()), None),
 
+                (Type::Nothing, _) => (Type::Nothing, None),
+                (_, Type::Nothing) => (Type::Nothing, None),
+
                 (Type::Any, _) => (Type::Bool, None),
                 (_, Type::Any) => (Type::Bool, None),
                 _ => {
@@ -351,6 +357,9 @@ pub fn math_result_type(
 
                 (Type::Any, _) => (Type::Bool, None),
                 (_, Type::Any) => (Type::Bool, None),
+
+                (Type::Nothing, _) => (Type::Nothing, None),
+                (_, Type::Nothing) => (Type::Nothing, None),
                 _ => {
                     *op = Expression::garbage(op.span);
                     (
@@ -378,6 +387,9 @@ pub fn math_result_type(
 
                 (Type::Any, _) => (Type::Bool, None),
                 (_, Type::Any) => (Type::Bool, None),
+
+                (Type::Nothing, _) => (Type::Nothing, None),
+                (_, Type::Nothing) => (Type::Nothing, None),
                 _ => {
                     *op = Expression::garbage(op.span);
                     (

crates/nu-protocol/src/value/mod.rs

@@ -2658,6 +2658,10 @@ impl Value {
             return lhs.operation(*span, Operator::Comparison(Comparison::LessThan), op, rhs);
         }
 
+        if matches!(self, Value::Nothing { .. }) || matches!(rhs, Value::Nothing { .. }) {
+            return Ok(Value::nothing(span));
+        }
+
         if !type_compatible(self.get_type(), rhs.get_type())
             && (self.get_type() != Type::Any)
             && (rhs.get_type() != Type::Any)
@@ -2697,6 +2701,10 @@ impl Value {
             );
         }
 
+        if matches!(self, Value::Nothing { .. }) || matches!(rhs, Value::Nothing { .. }) {
+            return Ok(Value::nothing(span));
+        }
+
         if !type_compatible(self.get_type(), rhs.get_type())
             && (self.get_type() != Type::Any)
             && (rhs.get_type() != Type::Any)
@@ -2734,6 +2742,10 @@ impl Value {
             );
         }
 
+        if matches!(self, Value::Nothing { .. }) || matches!(rhs, Value::Nothing { .. }) {
+            return Ok(Value::nothing(span));
+        }
+
         if !type_compatible(self.get_type(), rhs.get_type())
             && (self.get_type() != Type::Any)
             && (rhs.get_type() != Type::Any)
@@ -2771,6 +2783,10 @@ impl Value {
             );
         }
 
+        if matches!(self, Value::Nothing { .. }) || matches!(rhs, Value::Nothing { .. }) {
+            return Ok(Value::nothing(span));
+        }
+
         if !type_compatible(self.get_type(), rhs.get_type())
             && (self.get_type() != Type::Any)
             && (rhs.get_type() != Type::Any)

src/tests/test_math.rs

@@ -119,3 +119,62 @@ fn precedence_of_or_groups() -> TestResult {
 fn test_filesize_op() -> TestResult {
     run_test("-5kb + 4.5kb", "-500 B")
 }
+
+#[test]
+fn lt() -> TestResult {
+    run_test("1 < 3", "true").unwrap();
+    run_test("3 < 3", "false").unwrap();
+    run_test("3 < 1", "false")
+}
+
+// Comparison operators return null if 1 side or both side is null.
+// The motivation for this behaviour: JT asked the C# devs and they said this is
+// the behaviour they would choose if they were starting from scratch.
+#[test]
+fn lt_null() -> TestResult {
+    run_test("3 < null | to nuon", "null").unwrap();
+    run_test("null < 3 | to nuon", "null").unwrap();
+    run_test("null < null | to nuon", "null")
+}
+
+#[test]
+fn lte() -> TestResult {
+    run_test("1 <= 3", "true").unwrap();
+    run_test("3 <= 3", "true").unwrap();
+    run_test("3 <= 1", "false")
+}
+
+#[test]
+fn lte_null() -> TestResult {
+    run_test("3 <= null | to nuon", "null").unwrap();
+    run_test("null <= 3 | to nuon", "null").unwrap();
+    run_test("null <= null | to nuon", "null")
+}
+
+#[test]
+fn gt() -> TestResult {
+    run_test("1 > 3", "false").unwrap();
+    run_test("3 > 3", "false").unwrap();
+    run_test("3 > 1", "true")
+}
+
+#[test]
+fn gt_null() -> TestResult {
+    run_test("3 > null | to nuon", "null").unwrap();
+    run_test("null > 3 | to nuon", "null").unwrap();
+    run_test("null > null | to nuon", "null")
+}
+
+#[test]
+fn gte() -> TestResult {
+    run_test("1 >= 3", "false").unwrap();
+    run_test("3 >= 3", "true").unwrap();
+    run_test("3 >= 1", "true")
+}
+
+#[test]
+fn gte_null() -> TestResult {
+    run_test("3 >= null | to nuon", "null").unwrap();
+    run_test("null >= 3 | to nuon", "null").unwrap();
+    run_test("null >= null | to nuon", "null")
+}