[R][Client] Allow API response to be NULL

[joXemMx]

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Breaking up PR #18108 into smaller PRs.
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"))
}

Technical committee @Ramanth @saigiridhar21

[wing328]

lgtm. thanks for the PR