logind: fix emission of PropertiesChanged on seats

logind: fix emission of PropertiesChanged on seats

The story is the same as in 471cffcfb0e005b7c4044b3b52cc4f25d217efac:
device_attach() → seat_send_changed() → sd_bus_emit_properties_changed_strv()
→ emit_properties_changed_on_interface() → node_vtable_get_userdata()
→ seat_object_find(), which returns 0 because message == NULL.
But when we are emitting a signal, message is always NULL. Removing the
overeager check and assert in the called function allow the signal to be
emitted.

Fixes #13769.