Occasional test failure in test_plot_frequencies_time_series_with_taxa

[leehart]

FAILED tests/anoph/test_snp_frq.py::test_plot_frequencies_time_series_with_taxa[ag3_sim] - ValueError: No objects to concatenate

I suspect this is happening when random taxa are selected for testing, but somehow results in an empty dataset:

    # Pick a random taxon and taxa from valid taxa.
    sample_sets_taxa = (
        api.sample_metadata(sample_sets=sample_sets)["taxon"].dropna().unique().tolist()
    )
    taxon = random.choice(sample_sets_taxa)
    taxa = random.sample(sample_sets_taxa, random.randint(1, len(sample_sets_taxa)))

[leehart]

This failure is still occurring.

https://github.com/malariagen/malariagen-data-python/actions/runs/12240542636/job/34143612008

=================================== FAILURES ===================================
_____________ test_plot_frequencies_time_series_with_taxa[ag3_sim] _____________

fixture = <tests.anoph.conftest.Ag3Simulator object at 0x7f5ed6b178e0>
api = <malariagen_data.anoph.snp_frq.AnophelesSnpFrequencyAnalysis object at 0x7f5e913b8610>

    @parametrize_with_cases("fixture,api", cases=".")
    def test_plot_frequencies_time_series_with_taxa(
        fixture,
        api: AnophelesSnpFrequencyAnalysis,
    ):
        # Pick test parameters at random.
        all_sample_sets = api.sample_sets()["sample_set"].to_list()
        sample_sets = random.choice(all_sample_sets)
        site_mask = random.choice(api.site_mask_ids + (None,))
        transcript = random_transcript(api=api).name
        area_by = random.choice(["country", "admin1_iso", "admin2_name"])
        period_by = random.choice(["year", "quarter", "month"])
    
        # Pick a random taxon and taxa from valid taxa.
        sample_sets_taxa = (
            api.sample_metadata(sample_sets=sample_sets)["taxon"].dropna().unique().tolist()
        )
        taxon = random.choice(sample_sets_taxa)
        taxa = random.sample(sample_sets_taxa, random.randint(1, len(sample_sets_taxa)))
    
        # Compute SNP frequencies.
        ds = api.snp_allele_frequencies_advanced(
            transcript=transcript,
            area_by=area_by,
            period_by=period_by,
            sample_sets=sample_sets,
            min_cohort_size=1,  # Don't exclude any samples.
            site_mask=site_mask,
        )
    
        # Trim things down a bit for speed.
        ds = ds.isel(variants=slice(0, 100))
    
        # Plot with taxon.
>       fig = api.plot_frequencies_time_series(ds, show=False, taxa=taxon)

tests/anoph/test_snp_frq.py:1[565](https://github.com/malariagen/malariagen-data-python/actions/runs/12240542636/job/34143612008#step:5:566): 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
malariagen_data/util.py:1164: in check_types_wrapper
    return f(*args, **kwargs)
malariagen_data/anoph/snp_frq.py:1006: in plot_frequencies_time_series
    df_events = pd.concat(dfs, axis=0).reset_index(drop=True)
/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/pandas/core/reshape/concat.py:382: in concat
    op = _Concatenator(
/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/pandas/core/reshape/concat.py:445: in __init__
    objs, keys = self._clean_keys_and_objs(objs, keys)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pandas.core.reshape.concat._Concatenator object at 0x7f5e90baf970>
objs = [], keys = None

    def _clean_keys_and_objs(
        self,
        objs: Iterable[Series | DataFrame] | Mapping[HashableT, Series | DataFrame],
        keys,
    ) -> tuple[list[Series | DataFrame], Index | None]:
        if isinstance(objs, abc.Mapping):
            if keys is None:
                keys = list(objs.keys())
            objs_list = [objs[k] for k in keys]
        else:
            objs_list = list(objs)
    
        if len(objs_list) == 0:
>           raise ValueError("No objects to concatenate")
E           ValueError: No objects to concatenate

/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/pandas/core/reshape/concat.py:507: ValueError
[...]
=========================== short test summary info ============================
FAILED tests/anoph/test_snp_frq.py::test_plot_frequencies_time_series_with_taxa[ag3_sim] - ValueError: No objects to concatenate
================== 1 failed, 523 passed in 303.35s (0:05:03) ===================