Paraphase zips output vcf/vcf-index files only when these are created

[bagnacan]

This PR fixes a bug in the paraphase module.

Paraphase's output vcf/vcf-index files are created only if the provided input BAM file has reads in the analyzed region. When the analyzed region has no reads, paraphase creates no vcf/vcf-index files. For this reason, the paraphase module should zip these files only when they are available.
This PR introduces a check for the availability of these files, as well as a new test to verify that these outputs are indeed optional.
Test data needed to be extended to cover for this corner case (see test-dataset PR here).

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, include test data in your PR.