I see. OK that sounds like it could be a bug. Please post a support incident and we can have someone look into this problem.
The easiest solution for now is to use two update server behaviors. One to update all of the other fields and one to update the upload field. Then use the trigger of a successful uplaod in the second update so that the update of the file field doesn't take place if no file is specified.
I will look into the reported issue as a bug. Thank you for bringing it to my attention.