read_scikit_allel_vcfzarr returns string pad values instead of missing for INFO fields

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                                