Fix instant messaging attribution and tab group name in join flow

The instant message did not return the tab group metadata. The message
is only useful when the tab group is available, so skip sending the
message if the group is not found.
Fix the message user name to use either affected or triggered user name.
Fix the join flow to take the default tab group name if its not
available in preview data.

Bug: 385452227
Change-Id: I0e9054c039f27ae28fc6745562043ae994c252e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6126959
Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Siddhartha S <ssid@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1400630}