[Security Solution] Fixes ES|QL Tab resetting to KQL Bar (elastic#168716

)

## Summary

Handles elastic#168431

This PR fixes a race condition which was causing Discover ES|QL tab to
revert to it KQL query bar. See demo below:

| Before | After |
|---|---|
| <video
src="https://github.com/elastic/kibana/assets/7485038/24ff8ca9-30ce-4581-94d7-52908a2c81fd"
/>|<video
src="https://github.com/elastic/kibana/assets/7485038/746946ad-bd69-412f-bd38-d3d79b570c68"
/>|

(cherry picked from commit 6fd6966)

x-pack/plugins/security_solution/public/timelines/components/timeline/discover_tab_content/index.tsx

@@ -181,6 +181,10 @@ export const DiscoverTabContent: FC<DiscoverTabContentProps> = ({ timelineId })
     if (!savedObjectId) return;
     if (!status || status === 'draft') return;
     const latestState = getCombinedDiscoverSavedSearchState();
+    const index = latestState?.searchSource.getField('index');
+    /* when a new timeline is loaded, a new discover instance is loaded which first emits
+     * discover's initial state which is then updated in the saved search. We want to avoid that.*/
+    if (!index) return;
     if (!latestState || combinedDiscoverSavedSearchStateRef.current === latestState) return;
     if (isEqualWith(latestState, savedSearchById, savedSearchComparator)) return;
     debouncedUpdateSavedSearch(latestState, timelineId);