Do not all blank `sub` or `ses` in the custom transfer TUI

Currently in the TUI you can leave the 'subject' or 'session' box blank and it will run the transfer, and transfer nothing. This is very confusing, you would expect it to use 'all' in case the subject or session box is blank. If it is blank, we could convert to 'all' or raise an error saying these need to be filled it with some possible options ('all', 'all_sub', etc)