Client should be able to handle None on key values.

[zehawki]

Description

Client should be able to handle None on key values.

  File "/site-packages/typesense/documents.py", line 42, in upsert
    return self.api_call.post(self._endpoint_path(), document, params)
  File "/site-packages/typesense/api_call.py", line 145, in post
    timeout=self.config.connection_timeout_seconds)
  File "/site-packages/typesense/api_call.py", line 115, in make_request
    raise ApiCall.get_exception(r.status_code)(r.status_code, error_message)
typesense.exceptions.RequestMalformed: [Errno 400] Field `xyz` must be a string.

Steps to reproduce

Send a Python None value for some key in the call documents.upsert(document)

Expected Behavior

Gracefully handle None value for a key

Typsense Version: 0.21