test: Update test to expect UserWarning instead of ValueError for empty variant query results

The snp_frq.py code was updated to issue a UserWarning instead of raising
ValueError when a variant_query returns no results, but the corresponding
test was not updated. This fixes the test to use pytest.warns(UserWarning)
and verify that the returned dataset has zero variants.

Author: khushthecoder

tests/anoph/test_snp_frq.py

@@ -1520,24 +1520,27 @@ def test_allele_frequencies_advanced_with_variant_query(
         variant_query=variant_query,
     )
 
-    # Test a query that should fail.
+    # Test a query that should warn and return empty.
     variant_query = "effect == 'foobar'"
-    with pytest.raises(ValueError):
-        api.snp_allele_frequencies_advanced(
+    with pytest.warns(UserWarning):
+        ds_snp = api.snp_allele_frequencies_advanced(
             transcript=transcript,
             sample_sets=all_sample_sets,
             area_by=area_by,
             period_by=period_by,
             variant_query=variant_query,
         )
-    with pytest.raises(ValueError):
-        api.aa_allele_frequencies_advanced(
+        assert ds_snp.sizes["variants"] == 0
+
+    with pytest.warns(UserWarning):
+        ds_aa = api.aa_allele_frequencies_advanced(
             transcript=transcript,
             sample_sets=all_sample_sets,
             area_by=area_by,
             period_by=period_by,
             variant_query=variant_query,
         )
+        assert ds_aa.sizes["variants"] == 0
 
 
 @pytest.mark.parametrize("nobs_mode", ["called", "fixed"])