fix(fst): avoid infinite Z-scores when standard error is zero

Prevent division by zero from producing infinite values in Fst heatmaps.
Return NaN when SE is zero to avoid misleading visualization.

Refs malariagen#947

Author: adilraza99

malariagen_data/anoph/fst.py

@@ -539,11 +539,10 @@ def plot_pairwise_average_fst(
             if annotation == "standard error":
                 fig_df.loc[cohort1, cohort2] = se
             elif annotation == "Z score":
-                try:
-                    zs = fst / se
-                    fig_df.loc[cohort1, cohort2] = zs
-                except ZeroDivisionError:
+                if se == 0:
                     fig_df.loc[cohort1, cohort2] = np.nan
+                else:
+                    fig_df.loc[cohort1, cohort2] = fst / se
             else:
                 fig_df.loc[cohort1, cohort2] = fst