[Python] Bugfix in model.mustache

Before bugfix: a required and nullable attribute set to null/None causes a value error when received by the client.
After bugfix: This error is only caused for properties being required and not nullable
General info: The python client cannot distinguish between unset attributes (value=None) and attributes set to null (value=None)

Author: MalteEbner

modules/swagger-codegen/src/main/resources/python/model.mustache

@@ -99,8 +99,10 @@ class {{classname}}(object):
         :type: {{datatype}}
         """
 {{#required}}
+{{^nullable}}
         if self._configuration.client_side_validation and {{name}} is None:
             raise ValueError("Invalid value for `{{name}}`, must not be `None`")  # noqa: E501
+{{/nullable}}
 {{/required}}
 {{#isEnum}}
 {{#isContainer}}