added to_json method to DAEmpty class for JSON serialization

[jpagh]

I accidentally added a DAEmpty object to my interview dictionary, and now I can't "Show variables and values". I figured that since we added to_json() support that we might as well use it to avoid an error.

Error
TypeError: Object of type DAEmpty is not JSON serializable
Log
Traceback (most recent call last):
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/app.py", line 917, in full_dispatch_request
    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/app.py", line 902, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/docassemble/webapp/server.py", line 6712, in get_variables
    return jsonify(success=True, variables=variables, steps=steps, encrypted=is_encrypted, uid=session_id, i=yaml_filename)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/json/__init__.py", line 170, in jsonify
    return current_app.json.response(*args, **kwargs)  # type: ignore[return-value]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/json/provider.py", line 214, in response
    f"{self.dumps(obj, **dump_args)}\n", mimetype=self.mimetype
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/json/provider.py", line 179, in dumps
    return json.dumps(obj, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/lib/python3.12/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
  File "/usr/share/docassemble/local3.12/lib/python3.12/site-packages/flask/json/provider.py", line 121, in _default
    raise TypeError(f"Object of type {type(o).__name__} is not JSON serializable")
TypeError: Object of type DAEmpty is not JSON serializable