Fix runnables-related hickups (#8058)

* never error on absent/empty runnables file
* always activate terminal tab on runnable (re)schedule

Release Notes:

- N/A

crates/runnable/src/static_source.rs

@@ -79,7 +79,7 @@ impl<T: for<'a> Deserialize<'a> + PartialEq + 'static> TrackedFile<T> {
     ) -> Model<Self> {
         cx.new_model(move |cx| {
             cx.spawn(|tracked_file, mut cx| async move {
-                while let Some(new_contents) = tracker.next().await {
+                while let Some(new_contents) = tracker.next().await.filter(|s| !s.is_empty()) {
                     let Some(new_contents) = serde_json_lenient::from_str(&new_contents).log_err()
                     else {
                         continue;