Fix a problem with the keyfile backend whereby instantiating it with
the default NULL value would cause an assertion error:

https://gitlab.gnome.org/GNOME/glib/issues/1825

This issue showed up when running the test suites of uhttpmock and spice.

diff --git a/gio/gkeyfilesettingsbackend.c b/gio/gkeyfilesettingsbackend.c
--- a/gio/gkeyfilesettingsbackend.c
+++ b/gio/gkeyfilesettingsbackend.c
@@ -740,7 +740,8 @@ g_keyfile_settings_backend_set_property (GObject      *object,
     case PROP_FILENAME:
       /* Construct only. */
       g_assert (kfsb->file == NULL);
-      kfsb->file = g_file_new_for_path (g_value_get_string (value));
+      if (g_value_get_string (value))
+        kfsb->file = g_file_new_for_path (g_value_get_string (value));
       break;
 
     case PROP_ROOT_PATH: