[R][Client] Allow api responses to be NULL or invalid JSON and fix file_params for file upload

[joXemMx]

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Introduces changes:
ApiResponse now deserializes NULL values instead of producing errors by turning them into strings "NULL"

if (is.null(self$response)) {
  self$response <- charToRaw(jsonlite::toJSON("NULL"))
}

ApiClient now allows to deserialize responses not conforming to JSON, i.e. a simple string would normally produce an error in jsonlite::fromJSON but will now be passed on unchanged.

if (jsonlite::validate(raw_response)) {
  resp_obj <- jsonlite::fromJSON(raw_response)
} else {
  resp_obj <- raw_response
}

api.mustache file_params["{{baseName}}"] has been changed to file_params[["{{baseName}}"]] in order to function as intended when a file path is given as paramName in file_params[["{{baseName}}"]] <- curl::form_file(`{{paramName}}`)

Technical committee @Ramanth @saigiridhar21

[wing328]

thanks for the PR. Why would we allow invalid JSON in the server response?

I think current behaviour is to throw errors/exceptions when the server response is invalid.

[wing328]

I would recommend you to break down this PR into 3 smaller PRs for each fix/enhancement.

[joXemMx]

thanks for the PR. Why would we allow invalid JSON in the server response?

I think current behaviour is to throw errors/exceptions when the server response is invalid.

This was because my server response was a simple string, not a string in JSON format. But I now believe this is better to be fixed on the API side rather than in the Clients, so I will skip this in the separate merge requests.

[joXemMx]

Split into #18141 and #18142