text: Title: Enums should be nullable on input
  Body: Input fields deriving from Enum are not properly nullified. The resulting `str(mutation)` (prettified):
  
  ```graphql
  mutation {
    createUser(
      user: {
        firstName: "Jan"
        lastName: "Kowalski"
        middleName: null  // <-- this is string field, nullified correctly
        idType: None      // <-- BUG HERE: no value given to an enum should result in "null" instead
        idNumber: null
        // … snip
      }
    ) {
      // … snip
    }
  }
  ```
  
  Relevant code:
  
  ```python
  class IDType(sgqlc.types.Enum):
      __schema__ = admin_schema
      __choices__ = ('IDCard', 'Passport')
  
  class UserInput(sgqlc.types.Input):
      __schema__ = admin_schema
      __field_names__ = ('first_name', 'middle_name', 'last_name', 'id_type', 'id_number', ...)
      first_name = sgqlc.types.Field(sgqlc.types.non_null(String), graphql_name='firstName')
      last_name = sgqlc.types.Field(sgqlc.types.non_null(String), graphql_name='lastName')
      middle_name = sgqlc.types.Field(String, graphql_name='middleName')
      id_type = sgqlc.types.Field(IDType, graphql_name='idType')
      id_number = sgqlc.types.Field(String, graphql_name='idNumber')
      # ... snip
  ```
  
  Actual constructed values are `None`. It seems that the problem is in `sgqlc.types.EnumMeta.__to_graphql_input__` which should check `if value is None`. This change fixed the problem for me