Fixes serialization of array items in model_to_dict [python] (#9153)

* Adds tests of endpoint with inline and refed model

* Regen samples

* Samples regenerated

Author: spacether

.generator/templates/model_utils.mustache

@@ -1211,13 +1211,19 @@ def model_to_dict(model_instance, serialize=True):
                 # exist in attribute_map
                 attr = model_instance.attribute_map.get(attr, attr)
             if isinstance(value, list):
-                if not value or isinstance(value[0], PRIMITIVE_TYPES):
-                    # empty list or primitive types
-                    result[attr] = value
-                elif isinstance(value[0], ModelSimple):
-                    result[attr] = [x.value for x in value]
-                else:
-                    result[attr] = [model_to_dict(x, serialize=serialize) for x in value]
+               if not value:
+                   # empty list or None
+                   result[attr] = value
+               else:
+                   res = []
+                   for v in value:
+                       if isinstance(v, PRIMITIVE_TYPES) or v is None:
+                           res.append(v)
+                       elif isinstance(v, ModelSimple):
+                           res.append(v.value)
+                       else:
+                           res.append(model_to_dict(v, serialize=serialize))
+                   result[attr] = res
             elif isinstance(value, dict):
                 result[attr] = dict(map(
                     lambda item: (item[0],