handle empty files in venn diagram construction

[daler]

BEDTools raises an error -- file does not exist -- when trying to open an empty file. Venn diagram construction can create zero-length output files that are then sent to BEDTools to work on.

At least for the case of Venn diagrams, this should not be considered an error, since it's mostly outside the user's control. In fact, finding out whether there are zero jointly-shared features may be one reason why they're making a Venn diagram in the first place.

[daler]

Duplicate of #95 which has been fixed.