populate_clingen_alleles_for_variants doesn't handle case where Allele exists, but does not have ClinGen Allele

When this was 1st written, the only way an allele would be created was via ClinGen.

In a recent import, something went wrong (perhaps ClinGen was down?) and a lot of Alleles were created but w/o ClinGen.

If you call this function now, nothing happens as it skips over any alleles (whether they have ClinGen allele or not)

Need to handle the case where Allele is there but ClinGen is not.