When 0 reads remain after decontamination, an invalid gzip file is created

[bede]

Samtools considers an empty SAM/BAM file invalid, irritatingly. Workaround is to create empty but valid gzip file e.g.:

with gzip.open('empty.fastq.gz', 'wb') as f:
    pass

[bede]

Fixed in https://github.com/bede/hostile/commit/0b5ad56ed0d48b5d06a7ecc0a6d47b9db0ca41fc