keymap_ui: Don't try to parse empty action arguments as JSON (#36278)

Closes #ISSUE

Release Notes:

- Keymap Editor: Fixed an issue where leaving the arguments field empty
would result in an error even if arguments were optional

crates/settings_ui/src/keybindings.rs

@@ -2181,6 +2181,7 @@ impl KeybindingEditorModal {
 
         let value = action_arguments
             .as_ref()
+            .filter(|args| !args.is_empty())
             .map(|args| {
                 serde_json::from_str(args).context("Failed to parse action arguments as JSON")
             })