fix: fixes nan output in rca for specific case

In rca, if one group has 0 rows and the aggregation is mean,
we can get nan values in the output. This commit fixes that.

chaos_genius/core/rca/root_cause_analysis.py

@@ -360,6 +360,8 @@ def _get_waterfall_output_data(
 
         d1_agg = self._grp1_df[self._metric].agg(self._agg)
         d2_agg = self._grp2_df[self._metric].agg(self._agg)
+        d1_agg = 0 if pd.isna(d1_agg) else d1_agg
+        d2_agg = 0 if pd.isna(d2_agg) else d2_agg
         impact = d2_agg - d1_agg
         non_overlap_impact = df_subgroups["impact_non_overlap"].sum()