minor fix to skip calculate error metrics if external data is none

mlair/run_modules/post_processing.py

@@ -680,6 +680,7 @@ class PostProcessing(RunEnvironment):
             external_data = self._get_external_data(station, path)  # test data
 
             # test errors
+            if external_data is not None:
                 errors[station] = statistics.calculate_error_metrics(*map(lambda x: external_data.sel(type=x),
                                                                           [self.forecast_indicator, "obs"]),
                                                                      dim="index")