Fix migration from #40409 for users who haven't been migrated yet (#40916)

Closes #40874

Release Notes:

- Fixed an issue where migrating settings after v0.208.5+ would
spuriously enable prettier. This fix only affects those who have not
updated or migrated yet. For those who have already updated to version
v0.208.5 or later, placing `"formatter": []` in your settings in the
affected languages will fix the issue.

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/migrator/src/migrations/m_2025_10_16/settings.rs

@@ -58,7 +58,7 @@ fn restore_code_actions_on_format_inner(value: &mut Value, path: &[&str]) -> Res
             .map(|code_action| (code_action, Value::Bool(true))),
     );
 
-    obj.remove("formatter");
+    obj.insert("formatter".to_string(), Value::Array(vec![]));
     obj.insert(
         "code_actions_on_format".into(),
         Value::Object(code_actions_map),

crates/migrator/src/migrator.rs

@@ -2016,9 +2016,9 @@ mod tests {
                 &r#"{
                     "code_actions_on_format": {
                         "foo": true
-                    }
-                }
-                "#
+                    },
+                    "formatter": []
+                }"#
                 .unindent(),
             ),
         );
@@ -2053,6 +2053,7 @@ mod tests {
             .unindent(),
             Some(
                 &r#"{
+                    "formatter": [],
                     "code_actions_on_format": {
                         "foo": true,
                         "bar": true,
@@ -2080,6 +2081,7 @@ mod tests {
             .unindent(),
             Some(
                 &r#"{
+                    "formatter": [],
                     "code_actions_on_format": {
                         "foo": true,
                         "qux": true,