path exists: Empty path shouldn't be marked as exists (#11515)

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# Description
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Simply, `""` doesn't exist. 

It's easy to test the truthfulness of this:

```rust
fn main() {
    println!("{}", std::path::Path::new("").exists());
}
```
gives `false`

# User-Facing Changes
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helps us keep track of breaking changes. -->
Technically this is a breaking change, so:
- **breaking:** `path exists` no longer considers an empty path (`""`)
as exists

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crates/nu-command/src/path/exists.rs

@@ -130,6 +130,9 @@ If you need to distinguish dirs and files, please use `path type`."#
 }
 
 fn exists(path: &Path, span: Span, args: &Arguments) -> Value {
+    if path.as_os_str().is_empty() {
+        return Value::bool(false, span);
+    }
     let path = expand_path_with(path, &args.pwd);
     Value::bool(
         match path.try_exists() {