Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/activities/action_list.go

@@ -30,7 +30,7 @@ func (actions ActionList) getUserIDs() []int64 {
 
 func (actions ActionList) LoadActUsers(ctx context.Context) (map[int64]*user_model.User, error) {
 	if len(actions) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // returns nil when there are no actions
 	}
 
 	userIDs := actions.getUserIDs()