fix(frq): correctly validate max_len in plot_frequencies_heatmap

- handle max_len=0 correctly
- ensure ValueError is raised when expected
- preserves existing behavior for max_len=None
- fixes failing frequency heatmap tests

Author: adilraza99

malariagen_data/anoph/frq_base.py

@@ -227,7 +227,7 @@ def plot_frequencies_heatmap(
         **kwargs,
     ) -> plotly_params.figure:
         # Check len of input.
-        if max_len and len(df) > max_len:
+        if max_len is not None and len(df) > max_len:
             raise ValueError(
                 dedent(
                     f"""