When using V5 of the API, it is now possible to send nulls for object properties that should remain intact. However, when sending nulls or zeroes for bids, the system incorrectly sees the bid as less than floor price and you may see an error returned in the response message. We are hoping to resolve this issue this week.
It is ok to send nulls for other object properties in V5, as described in our MSDN API documentation :
V5 offers the ability to update campaigns, ads, ad groups, and keywords without requiring values for the properties that are not being changed. For example, you can specify new bid amounts for a keyword, but leave the Param1, Param2, Param3, Status, Text, and NegativeKeywords properties set to null when calling the UpdateKeywords method. Only the bid amounts will be changed. The keyword properties that are set to null will remain unchanged