VCF annotator should emit `.` for `missing` values in string-typed INFO fields

[theferrit32]

Labeling as a bug though it's not 100% clear that it is. pysam converts numeric types to the missing value . but not for string types. The VCF spec seems to suggest that is incorrect and . should be used. I removed the old manual mapping of None values to . because I assumed pysam would do it automatically for all the fields.

e.g. examples of INFO fields which may be written to output VCF files using the current VCF annotator in 2.2.0:

VRS_States=,AT
VRS_Lengths=1,.

This should be:

VRS_States=.,AT
VRS_Lengths=1,.

[theferrit32]

Am leaning towards it being a "bug" in pysam though, and that we should work around this appropriately in vrs-python to adhere to the spec.

The VCF v4.5 spec (https://samtools.github.io/hts-specs/VCFv4.5.pdf):
Zero length fields are not allowed, a dot (“.”) must be used instead.

As a non-pysam example, hail can serialize and deserialize both cases, like this:

'.,AT' -> [None, 'AT'] 

',AT' -> ['', 'AT']

So there seems to be inconsistency across common libraries.