gdataset: refactor check for old value in g_datalist_id_replace_data()

GData does not support to track NULL values.

That means, the existing value is NULL if-and-only-if the GDataElt is
NULL. Thus the check for `if (val == NULL && ...` is already covered by
`if (data)` above, and we can move that to the else branch.

glib/gdataset.c

@@ -1371,12 +1371,12 @@ g_datalist_id_replace_data (GData          **datalist,
             }
         }
     }
-
-  if (val == NULL && oldval == NULL && newval != NULL)
+  else
     {
-      if (datalist_append (&d, key_id, newval, destroy))
+      if (oldval == NULL && newval != NULL)
         {
-          set_d = TRUE;
+          if (datalist_append (&d, key_id, newval, destroy))
+            set_d = TRUE;
         }
     }