Handle zero-depth genotypes in allele-balance filtering

[thierrygosselin]

Summary

When -geno_minMM was enabled, genotype calling calculated the major/minor allele fraction by dividing by total genotype depth without checking for zero depth.

A zero-depth genotype therefore produced an undefined ratio rather than being explicitly treated as missing.

This change:

• detects zero total depth before calculating allele balance;
• marks zero-depth genotypes as missing;
• preserves the existing allele-balance threshold for genotypes with data.

This only affects the no-read edge case and does not change allele-balance results for genotypes with positive depth.