In v2 when calling updateCriteria with destinationUrl values of null or "", the result is actually those values respectively. In v3, null really means don't update destinationUrl, and "" means set the destinationUrl to null (i.e. clear the destinationUrl).
This oft requested change of semantics allows you to use updateCriteria in order to change maxCpc of a Keyword without needing to know the destinationUrl, which was the only issue that obliged developers to still use setKeywordListMaxCpc.
--Patrick Chanezon, AdWords API evangelist
