Fix T60383: mesh merge type callback returned NULL

Referenced by issue #60557, Merge to last and first disappear in the menus when using select box, select circle, and select lasso
Referenced by issue #60383, merge.merge keymap with type "at last" crashes blender when opening mesh(global) keymap tab in user prefs

source/blender/editors/mesh/editmesh_tools.c

@@ -2885,7 +2885,8 @@ static const EnumPropertyItem *merge_type_itemf(bContext *C, PointerRNA *UNUSED(
 		return item;
 	}
 
-	return NULL;
+	/* Get all items e.g. when creating keymap item. */
+	return merge_type_items;
 }
 
 void MESH_OT_merge(wmOperatorType *ot)