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>

services/actions/context.go

@@ -104,7 +104,7 @@ type TaskNeed struct {
 // FindTaskNeeds finds the `needs` for the task by the task's job
 func FindTaskNeeds(ctx context.Context, job *actions_model.ActionRunJob) (map[string]*TaskNeed, error) {
 	if len(job.Needs) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when the job has no needs
 	}
 	needs := container.SetOf(job.Needs...)