Nullable ID support in GetRowMatchingId?

@greg_smalter and @samrueby, Should we change the GetRowMatchingId methods so they take a nullable ID? We could have them always return null if null is passed, and this would be useful in PageInfo.init methods on add/edit pages as well as times when you have a nullable foreign key column.

I have never wished I had this feature, and I might not think to use it if it existed.

Could come in handy.