Validate empty samples when it is a single dot
Validate empty samples when it is a single dot

Freebayes sets the genotype as:
```
'.:.:.:.:.:.:.' 
```
when a sample is missing.
gusevfe/bioruby-vcf,gusevfe/bioruby-vcf,pjotrp/bioruby-vcf,gusevfe/bioruby-vcf,pjotrp/bioruby-vcf,pjotrp/bioruby-vcf