tests: Fall back if there's no seat_id for the current session_id

Such as in an ssh login.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3875

Fixes: 98b347b ("dbusmock/logind: Mock logind and remove KVM special handling")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4299>

src/tests/dbusmock-templates/logind.py

@@ -159,13 +159,17 @@ def GetSessionByPID(self, pid):
     return session_path
 
 def create_session(self, host_bus):
-    session_id = 'dummy'
-    seat_id = 'seat0'
+    session_id = None
+    seat_id = None
 
     if host_bus:
         session_id = find_host_session_id(host_bus)
         seat_id = find_host_seat_id(host_bus, session_id)
 
+    if not seat_id:
+        session_id = 'dummy'
+        seat_id = 'seat0'
+
     if not self.preferred_session_id or host_bus:
         self.preferred_session_id = session_id