[lib] Show "just you" for thread title if there are no members besides you

Summary:
This resolves ENG-2933, which was broken in D6615 when I removed the 'just you' special-case there.

Since filtering out the user (and pluralization) are now handled in getNameForThreadEntity, we need to check the length of the list there instead of in threadUIName now.

Test Plan: Make sure there are no empty strings in the web calendar filters, and that 'just you' appears where the empty strings previously did

Reviewers: inka, tomek, atul

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D6628