add validation: isFileUploadOrHasExistingValue

for situations where a user should be allowed to save a form without uploading a file, but only if the existing value in the database is not blank.

(my situation: a user has a profile form, including a profile image. They must upload an image - but if they've already uploaded one, they should be able to edit the other fields in the form and save, without having to upload another image)

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>