Allow empty ancestral alleles

[hyanwong]

Fixes #884

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7db1d38) 87.04% compared to head (edd8c29) 87.04%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #885   +/-   ##
=======================================
  Coverage   87.04%   87.04%           
=======================================
  Files           5        5           
  Lines        1767     1767           
  Branches      310      310           
=======================================
  Hits         1538     1538           
  Misses        140      140           
  Partials       89       89           

Flag	Coverage Δ
C	87.04% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hyanwong]

There's a bug here when calculating samples.num_alleles, which assumes that none of the alleles can be '""'.

[hyanwong]

Hmm, actually there's an even bigger problem here, right? We simply don't allow blank alleles ("") as allelic states in SGkit, right? In fact, the VCF spec says:

For simple insertions and deletions in which either the REF or one of the ALT alleles would otherwise be null/empty, the REF and ALT Strings must include the base before the event