fix: guard against empty allele count array in test_snp_allele_counts

When a randomly selected region has no SNP sites after applying the
site mask, the allele count array is empty and calling .max() on it
raises ValueError. Skip the max assertion for zero-size arrays.

This fixes a pre-existing flaky test failure:
  ValueError: zero-size array to reduction operation maximum which has no identity

Author: khushthecoder

tests/anoph/test_snp_data.py

@@ -1134,7 +1134,8 @@ def check_snp_allele_counts(
     assert ac.shape == (pos.shape[0], 4)
     assert np.all(ac >= 0)
     an = ac.sum(axis=1)
-    assert an.max() <= 2 * n_samples
+    if an.size > 0:
+        assert an.max() <= 2 * n_samples
 
     # Run again to ensure loading from results cache produces the same result.
     ac2 = api.snp_allele_counts(