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/repository/files/update.go

@@ -510,7 +510,7 @@ func modifyFile(ctx context.Context, t *TemporaryUploadRepository, file *ChangeR
 	}
 
 	if writeObjectRet.LfsContent == nil {
-		return nil, nil // No LFS pointer, so nothing to do
+		return nil, nil //nolint:nilnil // No LFS pointer, so nothing to do
 	}
 	defer writeObjectRet.LfsContent.Close()