Exception "TypeError: 'NoneType' object is not iterable" if type with allOf does not has any additional properties

[turtletramp]

When using a type that derives from a base type that does not have any additional properties compared to the base type an exception happens.
Swagger definition that can trigger the issue (where OrderLineBase is of course a valid type):

    "OrderLineModify": {
      "allOf": [
        {
          "$ref": "#/definitions/OrderLineBase"
        },
        {
          "type": "object",
          "additionalProperties": false
        }
      ]

This is the exception

Traceback (most recent call last):
  File "/usr/local/bin/swg2rst", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger2rst.py", line 51, in main
    swagger_doc = doc_module.SwaggerObject(doc, examples=examples)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/base_swagger_object.py", line 71, in __init__
    self._fill_schemas_from_definitions(obj)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/base_swagger_object.py", line 125, in _fill_schemas_from_definitions
    definition, name, SchemaTypes.DEFINITION, root=self)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/schema_objects.py", line 28, in create_schema
    schema = Schema(obj, schema_type, storage=cls, name=name, root=root)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/schema.py", line 52, in __init__
    self._parse_all_of_property(obj)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/schema.py", line 114, in _parse_all_of_property
    schema if schema else self.storage.get(_id)
  File "/usr/local/lib/python3.6/site-packages/swg2rst/swagger/schema_objects.py", line 90, in merge_schemas
    for _prop in _schema.properties:
TypeError: 'NoneType' object is not iterable