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/issue/assignee.go

@@ -234,7 +234,7 @@ func TeamReviewRequest(ctx context.Context, issue *issues_model.Issue, doer *use
 	}
 
 	if comment == nil || !isAdd {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil because no comment was created or it is a removal
 	}
 
 	return comment, teamReviewRequestNotify(ctx, issue, doer, reviewer, isAdd, comment)

services/issue/commit.go

@@ -113,7 +113,7 @@ func getIssueFromRef(ctx context.Context, repo *repo_model.Repository, index int
 	issue, err := issues_model.GetIssueByIndex(ctx, repo.ID, index)
 	if err != nil {
 		if issues_model.IsErrIssueNotExist(err) {
-			return nil, nil
+			return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 		}
 		return nil, err
 	}

services/issue/issue.go

@@ -229,7 +229,7 @@ func AddAssigneeIfNotAssigned(ctx context.Context, issue *issues_model.Issue, do
 	}
 	if isAssigned {
 		// nothing to do
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil because the user is already assigned
 	}
 
 	valid, err := access_model.CanBeAssigned(ctx, assignee, issue.Repo, issue.IsPull)