fixes for standardize=None case

Author: vpratz

bayesflow/approximators/continuous_approximator.py

@@ -59,7 +59,7 @@ def __init__(
         if isinstance(standardize, str) and standardize != "all":
             self.standardize = [standardize]
         else:
-            self.standardize = standardize
+            self.standardize = standardize or []
 
         if self.standardize == "all":
             # we have to lazily initialize these

bayesflow/approximators/model_comparison_approximator.py

@@ -58,7 +58,7 @@ def __init__(
         if isinstance(standardize, str) and standardize != "all":
             self.standardize = [standardize]
         else:
-            self.standardize = standardize
+            self.standardize = standardize or []
 
         if self.standardize == "all":
             # we have to lazily initialize these
@@ -333,6 +333,7 @@ def get_config(self):
             "adapter": self.adapter,
             "classifier_network": self.classifier_network,
             "summary_network": self.summary_network,
+            "standardize": self.standardize,
         }
 
         return base_config | serialize(config)