RNA: correct enum flag callback

Was returning NULL, auto-completing:

    bpy.types.Material.bl_rna.properties["type"].

Would crash, accessing 'default_flag', thanks to @lukastoenne for investigating.

source/blender/makesrna/intern/rna_rna.c

@@ -813,7 +813,7 @@ static EnumPropertyItem *rna_EnumProperty_default_itemf(bContext *C, PointerRNA
 
 	/* incompatible default attributes */
 	if ((prop_parent->flag & PROP_ENUM_FLAG) != (prop->flag & PROP_ENUM_FLAG)) {
-		return NULL;
+		return DummyRNA_NULL_items;
 	}
 
 	if ((eprop->itemf == NULL) ||