fix(core): replace missing yhats with None/null

In some cases `df[col`] can be `np.nan`, so explicitly replace those
values with None.

The type checker does not like this for some reason.

chaos_genius/core/utils/round.py

@@ -63,5 +63,5 @@ def round_column_in_df(df: pd.DataFrame, col: str):
                 ),
             ),
         ),
-        df[col]
+        None,
     )