Add `_is_null` and `_is_not_null` operators

[zlargon]

This PR is related to #682 and #823.

• Add _is_null and _is_not_null operators.
• Add test cases.
• Update README.

example

db.todos = [
  { id: 1, completeDate: null },
  { id: 2, completeDate: null },
  { id: 3, completeDate: '2022-11-20T13:02:47.210Z' },
  { id: 4, completeDate: '2023-12-05T17:58:17.420Z' },
]

attr_is_null

// GET /todos?completeDate_is_null
[
  { id: 1, completeDate: null },
  { id: 2, completeDate: null }
]

attr_is_not_null

// GET /todos?completeDate_is_not_null
[
  { id: 3, completeDate: '2022-11-20T13:02:47.210Z' },
  { id: 4, completeDate: '2023-12-05T17:58:17.420Z' },
]