[Code Health] Add an extra helper isEmpty for ListModels

Create a helper method to check if the list model item array is empty.

Bug: 363324216
Change-Id: I326d48456b3f26d7835f6105759e991f6edf3e3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5829712


Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Brandon Fong <bjfong@google.com>
Cr-Commit-Position: refs/heads/main@{#1349487}