user_status: Fix bug with resetting away status.

The fix is pretty simple here--if the client
doesn't send an away status, then don't change
it.

I improved the tests to cover this case.

Fixes #17071

zerver/lib/actions.py

@@ -4183,7 +4183,9 @@ def do_update_user_status(user_profile: UserProfile,
                           away: Optional[bool],
                           status_text: Optional[str],
                           client_id: int) -> None:
-    if away:
+    if away is None:
+        status = None
+    elif away:
         status = UserStatus.AWAY
     else:
         status = UserStatus.NORMAL