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/gitdiff/csv.go

@@ -193,7 +193,7 @@ func createCsvDiff(diffFile *DiffFile, baseReader, headReader *csv.Reader) ([]*T
 		}
 		if aRow == nil && bRow == nil {
 			// No content
-			return nil, nil
+			return nil, nil //nolint:nilnil // return nil to indicate that the row has no content
 		}
 
 		aIndex := 0      // tracks where we are in the a2bColMap