code: def null(self, field, **validations):
          
          values = []
          for found in self._find_by_field(field):
              reality = found["reality"]
              schema = {"type": "null"}
              skip = self._input_boolean(validations.pop("skip", False))
              if not skip:
                  self._assert_schema(schema, reality)
              values.append(reality)
          return values
docs: *Asserts the field as JSON null.*
  
          The field consists of parts separated by spaces, the parts being
          object property names or array indices starting from 0, and the root
          being the instance created by the last request (see `Output` for it).
  
          For asserting deeply nested properties or multiple objects at once,
          [http://goessner.net/articles/JsonPath|JSONPath] can be used with
          [https://github.com/h2non/jsonpath-ng#jsonpath-syntax|supported JSONPath expressions],
          the root being the response body.
  
          *Validations*
  
          The JSON Schema validation keywords
          [https://json-schema.org/understanding-json-schema/reference/generic.html|common for all types]
          can be used. Validations are optional but update the schema of
          the property (more accurate) if given.
          `Output Schema` can be used for the current schema of the field.
  
          The keyword will fail if any of the given validations fail.
          Given validations can be skipped altogether by adding ``skip=true``.
          When skipped, the schema is updated but the validations are not ran.
          Skip is intented mainly for debugging the updated schema before aborting.
  
          *Examples*
  
          | `PUT`  | /users/1 | { "deactivated_at": null } | # https://jsonplaceholder.typicode.com/users/1 |
          | `Null` | response body deactivated_at | |
          | `Null` | $.deactivated_at | | # JSONPath alternative |