Fixed the if condition with flake8 requirement

Author: yasinrawther

rest_framework_docs/api_endpoint.py

@@ -123,7 +123,7 @@ def __get_serializer_fields__(self, serializer):
                     "sub_fields": sub_fields,
                     "required": field.required,
                     "to_many_relation": to_many_relation,
-                    "max_length": (field.__dict__.get('max_length', '-') if field.__dict__.get('max_length', '-') != None else '-')
+                    "max_length": (field.__dict__.get('max_length', '-') if field.__dict__.get('max_length', '-') is not None else '-')
                 })
             # FIXME:
             # Show more attibutes of `field`?