Cream grid jobs fail when no input or output files to transfer. #3415

Fix TransferRequest to behave intelligently when created with an empty
list of transfers. Before, it would issue a transfer command with "NULL"
for source and destination, which fails, causing Cream jobs to go on
hold.
Docs for cream file transfer bug. #3415
Merge branch 'V7_8-branch' into V7_9_3-branch