fix no entry found for key error when GT is missing in common_variant…

[hxj5]

when some sample's FORMAT is .:.:.:.:.:. in common_variants_covered.vcf, the HashMap of record.call[sample] is empty, which would cause 'no entry found for key' error for record.call[sample]["GT"][0].

[katkoad]

Im having this problem and this fork was never merged.