Don't call g_variant_store() with NULL

A GVariant may require 0 bytes to serialize. In which case, `data_size`
will be 0 and we'll call `g_malloc0 (0)` which returns `NULL`.

The `data` argument of `g_variant_store` is declared as non-nullable,
so we shouldn't pass it `NULL`. In older glib versions, this wasn't
explicitly checked and it still transparently worked for our case
because there was nothing to actually serialize. Starting from v2.84.1,
a precondition was added:

https://gitlab.gnome.org/GNOME/glib/-/commit/39c05b13123b12622ee2c93c170dbf20b573f6ac

Let's respect the API here and stop calling the function if it's not
needed.

Note that `g_variant_new_from_data()` supports being passed `NULL` and 0
for the data and size arguments.

Author: jlebon

modulemd/modulemd-util.c

@@ -363,9 +363,13 @@ modulemd_variant_deep_copy (GVariant *variant)
 {
   const GVariantType *data_type = g_variant_get_type (variant);
   gsize data_size = g_variant_get_size (variant);
-  gpointer data = g_malloc0 (data_size);
+  gpointer data = NULL;
 
-  g_variant_store (variant, data);
+  if (data_size > 0)
+    {
+      data = g_malloc0 (data_size);
+      g_variant_store (variant, data);
+    }
 
   return g_variant_ref_sink (g_variant_new_from_data (
     data_type, data, data_size, FALSE, destroy_variant_data, data));