Handle the scenario that no fusions are detected by TopHat-Fusion

[amatchan]

When TopHat-Fusion is run on a RNA-Seq sample, it usually finds thousands of candidate fusions, even in normal, supposedly healthy samples. The tophat-fusion-post step then uses the output fusion list and a check is made to ensure the file exists and isn't empty.
Technically it might be that no fusions have been detected and the pipeline should be able to handle this and report an appropriate output message to the user.