[TabGroupSync] Explicitly return nullptr for OTR profiles

Ensures we return nullptr for the TabGroupServiceWrapper for profiles
that are off the record (Ex: Incognito, Guest Mode). This will prevent
crashes / invalid access when we try to call this function in these
cases.

Change-Id: I6d95bc56828bc07c265cae6312ca1a65432a1ff3
Bug: 350514491, 356770826
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5760277
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Darryl James <dljames@chromium.org>
Reviewed-by: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1336795}