text: Title: Default value for custom input object type
  Body: I have a custom input object type
  
  ```
  class Filters(graphene.InputObjectType):                                        
      name = graphene.String()
      type = graphene.List(graphene.String)
  ```
  
  and the query definition
  
  ```
  class Query(graphene.ObjectType):                                               
      search = graphene.Field(
          SearchResult, query=graphene.String(required=True),
          filters=graphene.Argument(Filters, default_value=None)
      )
  ```
  
  I want to be able to provide an empty default value for the `filters` argument. With the above definition, the argument is not set as optional and I'm getting this error message:
  
  ```
  {
    "errors": [
      {
        "message": "resolve_search() missing 1 required positional argument: 'filters'",
        "locations": [
          {
            "line": 15,
            "column": 3
          }
        ],
        "path": [
          "search"
        ]
      }
    ],
    "data": {
      "search": null
    }
  }
  ```
  
  What is the correct way to provide an empty default value for the argument of custom object type? I'm at these package versions:
  
  `graphene==2.1.3`
  `graphql-core==2.1`
  `graphql-server-core==1.1.1`