code: def _validate_collections_have_default_values(class_name, property_name, property_descriptor):
      
      # We don't want properties of collection type having "null" values, since that may cause
      # unexpected errors during GraphQL query execution and other operations.
      if property_descriptor.type_id in COLLECTION_PROPERTY_TYPES:
          if property_descriptor.default is None:
              raise IllegalSchemaStateError(u'Class "{}" has a property "{}" of collection type with '
                                            u'no default value.'.format(class_name, property_name))
docs: Validate that if the property is of collection type, it has a specified default value.