This is caused by r58921 which I introduced the wrong V8 generator code.
Created attachment 55613 [details] Proposed Patch
Comment on attachment 55613 [details] Proposed Patch WebCore/bindings/scripts/CodeGeneratorV8.pm: + return "v8StringOrNull(exec, $value)" if $conv; Oh, sorry. I should have caught that in review. :(
Committed as http://trac.webkit.org/changeset/59104.