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/issues/issue.go

@@ -498,7 +498,7 @@ func (issue *Issue) GetLastComment(ctx context.Context) (*Comment, error) {
 		return nil, err
 	}
 	if !exist {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return &c, nil
 }

models/issues/review.go

@@ -384,7 +384,7 @@ func CreateReview(ctx context.Context, opts CreateReviewOptions) (*Review, error
 // GetCurrentReview returns the current pending review of reviewer for given issue
 func GetCurrentReview(ctx context.Context, reviewer *user_model.User, issue *Issue) (*Review, error) {
 	if reviewer == nil {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when reviewer is nil
 	}
 	reviews, err := FindReviews(ctx, FindReviewOptions{
 		Types:      []ReviewType{ReviewTypePending},