[SingleLineView] Fallback to mUser when senderPerson is null

Bug: 371843309
Test: Send group notification for conversations. Last message sender in one of these conversations should be null. Check SingleLineView and see Messaging#mUser name comes instead of null
Flag: com.android.systemui.notification_async_hybrid_view_inflation
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:29b080800b12dd5b406a1799150bd39d46d75618)
Merged-In: I011378b8f0f0a29557b651fb85273a8cc91db6b8
Change-Id: I011378b8f0f0a29557b651fb85273a8cc91db6b8