Searching: Return an empty result if the search term is an empty string.

This mimics the behaviour we get from synapse as, this fixes
https://github.com/vector-im/riot-web/issues/12715.

src/Searching.js

@@ -87,6 +87,13 @@ async function localSearch(searchTerm, roomId = undefined) {
         searchArgs.room_id = roomId;
     }
 
+    const emptyResult = {
+        results: [],
+        highlights: [],
+    };
+
+    if (searchTerm === "") return emptyResult;
+
     const eventIndex = EventIndexPeg.get();
 
     const localResult = await eventIndex.search(searchArgs);
@@ -97,11 +104,6 @@ async function localSearch(searchTerm, roomId = undefined) {
         },
     };
 
-    const emptyResult = {
-        results: [],
-        highlights: [],
-    };
-
     const result = MatrixClientPeg.get()._processRoomEventsSearch(
         emptyResult, response);