func optionalKeyValueJSON() {

    let jsonStr1 = #"[{"key1":"value1", "key2": "value2"}]"#
    let jsonStr2 = #"[{"key2": "value2"}]"#
    let jsonStr3 = #"[{"key1": null, "key2": "value2"}]"#

    struct SCodable: Codable {
        let key1: String?
        let key2: String
    }

    func decode(jsonStr: String) {
        do {
            let decoded = try JSONDecoder().decode([SCodable].self, from: Data(jsonStr.utf8))
            print("Success: \(decoded)")
        } catch {
            print("Error: \(error)")
        }
    }

    decode(jsonStr: jsonStr1) //Works with key1 optional
    decode(jsonStr: jsonStr2) //Works with key1 optional while key1 is not present in the JSON
    decode(jsonStr: jsonStr3) //Works with key1 optional while key1 is null, ie ~ nil
}
optionalKeyValueJSON()

Outputs:

$>Success: [SCodable(key1: Optional("value1"), key2: "value2")]
$>Success: [SCodable(key1: nil, key2: "value2")]
$>Success: [SCodable(key1: nil, key2: "value2")]