read_scikit_allel_vcfzarr returns string pad values instead of missing for INFO fields

[jeromekelleher]

For example, in the test test_vcf_scikit_allel.py::test_all_fields, the variant_AA field is reported as ['' '' '' '' 'T' 'T' 'G' '' ''] instead of ['.' '.' '.' '.' 'T' 'T' 'G' '.' '.']

I'm not sure if this generalises to other fields, just reporting test breakage seen while fixing #1192