settings: fix wrong assertion in keyfiles _storages_consolidate()

The storage may also contain a tombstone, and have no connection to steal.

src/settings/plugins/keyfile/nms-keyfile-plugin.c

@@ -516,7 +516,6 @@ _storages_consolidate (NMSKeyfilePlugin *self,
 		}
 
 		connection = nms_keyfile_storage_steal_connection (storage);
-		nm_assert (NM_IS_CONNECTION (connection));
 
 		callback (NM_SETTINGS_PLUGIN (self),
 		          NM_SETTINGS_STORAGE (storage),

src/settings/plugins/keyfile/nms-keyfile-storage.c

@@ -55,7 +55,8 @@ NMConnection *
 nms_keyfile_storage_steal_connection (NMSKeyfileStorage *self)
 {
 	nm_assert (NMS_IS_KEYFILE_STORAGE (self));
-	nm_assert (NM_IS_CONNECTION (self->connection));
+	nm_assert (   (!self->connection && self->is_tombstone)
+               || NM_IS_CONNECTION (self->connection));
 
 	return g_steal_pointer (&self->connection);
 }