Hack for avoiding Galaxy issue with selecting zero files.

See Trello issue https://trello.com/c/lp5YmA1O and discussion
http://lists.bx.psu.edu/pipermail/galaxy-dev/2014-February/018366.html

Hack based on trick from one of Bjoern's wrappers (macs2_callpeak.xml)
suggested by John Chilton.