Make the VCF option in CollectSamErrorMetrics optional.

[nh13]

Description

In many cases, were re-sequencing a sample that does not differ from the reference genome (ex. E. Coli). I would like to run CollectSamErrorMetrics without having to give an empty VCF.

Funnily enough, this was internally supported, but the VCF option was never set to be optional. Just see the initializeVcfDataSource method to verify.

Checklist

Content

• Added or modified tests to cover changes and any new functionality
• Edited the README / documentation (if applicable)
• All tests passing on Travis

Review

• Final thumbs-up from reviewer
• Rebase, squash and reword as applicable

For more detailed guidelines, see https://github.com/broadinstitute/picard/wiki/Guidelines-for-pull-requests

[pshapiro4broad]

Please fix these minor formatting issues, looks good otherwise.