`add_document` method doesn't give access to `missing_payload` error when empty string is send

[alallema]

Description
When trying to add an empty string document in Meilisearch the error message should be:

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

Expected behavior
Receive a formal message for this case from Meilisearch API.

Current behavior
The following error message is received when the method is used in this way:

client.index('movies').add_documents('')

{
	"message": "The 'json' payload provided is malformed. Couldn't serialize document value: data did not match any variant of untagged enum Either.",
	"code": "malformed_payload",
	"type": "invalid_request",
	"link": "https://docs.meilisearch.com/errors#malformed_payload"
}

Environment (please complete the following information):

• Meilisearch version: v.0.29.1
• meilisearch-python version: v0.22.1

[adripo]

I would like to work on this issue. I read your contribution guidelines and you prefer to not assign people to issue, but just wanted to let you know that I am working on this.

[alallema]

Hi @adripo,
Great, thank you for letting us know 😊

[adripo]

Hi @alallema I fixed the issue. It was a bit complicated to find out how the flow works. I had to find out how the api requests are managed in the main repo (and I don't know rust 😓)

[alallema]

My bad the issue may not have been clear enough, but the problem was only that the SDK did not send an empty string correctly when it should have