Fix: Handle empty checkOnSave/target values

This fixes a regression introduced by #13290, in which failing to set
`checkOnSave/target` (or `checkOnSave/targets`) would lead to an invalid
config.

crates/rust-analyzer/src/config.rs

@@ -182,7 +182,7 @@ config_data! {
         /// `["aarch64-apple-darwin", "x86_64-apple-darwin"]`.
         ///
         /// Aliased as `"checkOnSave.targets"`.
-        checkOnSave_target | checkOnSave_targets: CheckOnSaveTargets           = "[]",
+        checkOnSave_target | checkOnSave_targets: Option<CheckOnSaveTargets>           = "null",
 
         /// Toggles the additional completions that automatically add imports when completed.
         /// Note that your client must specify the `additionalTextEdits` LSP client capability to truly have this feature enabled.
@@ -1153,10 +1153,15 @@ impl Config {
             }
             Some(_) | None => FlycheckConfig::CargoCommand {
                 command: self.data.checkOnSave_command.clone(),
-                target_triples: match &self.data.checkOnSave_target.0[..] {
-                    [] => self.data.cargo_target.clone().into_iter().collect(),
-                    targets => targets.into(),
-                },
+                target_triples: self
+                    .data
+                    .checkOnSave_target
+                    .clone()
+                    .and_then(|targets| match &targets.0[..] {
+                        [] => None,
+                        targets => Some(targets.into()),
+                    })
+                    .unwrap_or_else(|| self.data.cargo_target.clone().into_iter().collect()),
                 all_targets: self.data.checkOnSave_allTargets,
                 no_default_features: self
                     .data
@@ -2126,8 +2131,11 @@ fn field_props(field: &str, ty: &str, doc: &[&str], default: &str) -> serde_json
                 "The command will be executed in the project root."
             ],
         },
-        "CheckOnSaveTargets" => set! {
+        "Option<CheckOnSaveTargets>" => set! {
             "anyOf": [
+                {
+                    "type": "null"
+                },
                 {
                     "type": "string",
                 },

docs/user/generated_config.adoc

@@ -190,7 +190,7 @@ cargo check --workspace --message-format=json --all-targets
 ```
 .
 --
-[[rust-analyzer.checkOnSave.target]]rust-analyzer.checkOnSave.target (default: `[]`)::
+[[rust-analyzer.checkOnSave.target]]rust-analyzer.checkOnSave.target (default: `null`)::
 +
 --
 Check for specific targets. Defaults to `#rust-analyzer.cargo.target#` if empty.

editors/code/package.json

@@ -640,8 +640,11 @@
                 },
                 "rust-analyzer.checkOnSave.target": {
                     "markdownDescription": "Check for specific targets. Defaults to `#rust-analyzer.cargo.target#` if empty.\n\nCan be a single target, e.g. `\"x86_64-unknown-linux-gnu\"` or a list of targets, e.g.\n`[\"aarch64-apple-darwin\", \"x86_64-apple-darwin\"]`.\n\nAliased as `\"checkOnSave.targets\"`.",
-                    "default": [],
+                    "default": null,
                     "anyOf": [
+                        {
+                            "type": "null"
+                        },
                         {
                             "type": "string"
                         },