return g_variant_new_maybe for NULL variants

From glib 2.84.1, `g_variant_store` will not accept `NULL` values
instead failing an assertion and throwing a critical error.

In the case where `variant` == `NULL`, instead we can wrap and return
a `g_variant_new_maybe` with a child of `NULL` to achieve the same
effective result.

Closes: #623
Signed-off-by: Matt Jolly <[email protected]>

Author: Kangie

modulemd/modulemd-util.c

@@ -361,6 +361,12 @@ destroy_variant_data (gpointer data)
 GVariant *
 modulemd_variant_deep_copy (GVariant *variant)
 {
+  // g_variant_store will not accept NULL from 2.84.1
+  if (variant == NULL)
+    {
+      return g_variant_ref_sink (g_variant_new_maybe (G_VARIANT_TYPE_VARIANT, NULL));
+    }
+
   const GVariantType *data_type = g_variant_get_type (variant);
   gsize data_size = g_variant_get_size (variant);
   gpointer data = g_malloc0 (data_size);