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>

modules/git/foreachref/parser.go

@@ -100,7 +100,7 @@ func (p *Parser) Err() error {
 func (p *Parser) parseRef(refBlock string) (map[string]string, error) {
 	if refBlock == "" {
 		// must be at EOF
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to signal EOF
 	}
 
 	fieldValues := make(map[string]string)