More readable error when empty doc list passed

[dipanshunagar]

Raising the error here in the python wrapper itself also saves the server a wasted request, while being more meaningful to the python developer.

Current behaviour when empty document list passed to index.add_documents:
meilisearch.errors.MeiliSearchApiError: MeiliSearchApiError. Error code: malformed_payload. Error message: The 'json' payload provided is malformed. Couldn't serialize document value: data did not match any variant of untagged enum Either. Error documentation: https://docs.meilisearch.com/errors#malformed_payload Error type: invalid_request

Proposed behaviour:
meilisearch.errors.MeiliSearchError: MeiliSearchError. Error message: An empty document list was provided. No request will be sent to the Meilisearch API

[alallema]

Hi @dipanshunagar,
Thank you very much for raising this issue and creating this PR.
Indeed, there is a problem the return of an empty document should be :

{
	"message": "A json payload is missing.",
	"code": "missing_payload",
	"type": "invalid_request",
	"link": "https://docs.meilisearch.com/errors#missing_payload"
}

This should be fixed.
For the rest you are right, sending a document sends a useless request but in the functioning of all SDKs in general we chose to be always closer to the behavior of the API so that all error messages are relevant and the user has access to the real messages of the API.

So I'm going to open an issue about an empty document not being sent correctly by the add_document method and if you want to modify your PR to fix that, that would be great.
But we still want to get the error message from the API first if it's available.

Edit: #585

[alallema]

Hi @dipanshunagar,
Thank you again for your contribution, I open an issue about your PR so I closed this one. Feel free to open a new one