GlobalSearch renders unnecessarily when not empty

[taneliang]

Describe the bug

A clear and concise description of what the bug is.

To Reproduce

Steps to reproduce the behavior:

1. Start NUSMods locally (you can't use nusmods.com as we need a development copy of React).

2. Type something into global search.

3. Begin recording a profile in the React Profiler (Chrome version here).

4. Do something unrelated to global search, e.g. go to http://localhost:8080/contribute and click the Show More button in the Map the School section.

5. Stop the recording.

6. Notice GlobalSearch rendered:
   

7. Reload the page, and record another profile without entering anything into global search. Notice that GlobalSearch did not render and that there were fewer renders:
   

Expected behavior

GlobalSearch should not render unnecessarily.

Additional context

It may be easier to fix this after #3039 is merged, so that we can use the React scheduling profiler to see if there are any unexpected state updates.