rnaSPAdes sometimes does not emit transcripts.fasta

[dpark01]

Apparently in some scenarios, the SPAdes assembler in RNA mode does not output a transcripts.fasta file at all. Not sure if this is it's normal behavior in empty output scenarios (no output file instead of 0-byte output file). In any case, we should tolerate this behavior: currently, this results in an error/failure. We should probably interpret it as empty output and simply pass along empty output successfully.

[tomkinsc]

Potentially of note is that a newer version of SPAdes has been released, 3.12.0:
https://github.com/ablab/spades/releases/tag/v3.12.0