Handle empty SNP/AA results in allele frequency tests

[adilraza99]

Summary

Fix a transient failure in allele frequency tests caused by random parameter combinations producing empty results.

Changes

• add guards for ValueError when random parameters yield:
  • no variant SNPs
  • no amino acid change SNPs
• update two tests:
  • test_allele_frequencies_with_str_cohorts_and_sample_query
  • test_allele_frequencies_with_str_cohorts_and_sample_query_options

Rationale

The functions snp_allele_frequencies() and aa_allele_frequencies() correctly raise
ValueError when no variants remain after filtering. Randomized test inputs can
legitimately trigger this case, causing intermittent failures.

The tests now return early when this situation occurs, following the existing
pattern already used elsewhere in the test suite.

Impact

No changes to library behaviour.
This only stabilizes the test suite by handling valid edge cases.

Closes #1064