Fix optional file fields with RB.BaseResource.

In change 624e56e I gave BaseResource the ability to upload multiple files as
part of an API request. This worked, but wasn't able to cope with file fields
that are optional. In the case where one or more of the fields pointed to by
payloadFileKeys was undefined, it would raise a TypeError.

Testing done: Used this in conjunction with my pre-commit WIP, which can
sometimes upload only one file to a resource that takes two.