Extract a non-existing key from the json string doesn't seem to be working properly

[rohitsivakumar]

I tried out your library and it works good considering the keys in the json string exist and are correctly provided in the jsonExtract function. However, trying to send a non-existing key results in an incorrectly parsed string instead of returning an empty string. I think in this scenario of non-key existence case it should provide some way to indicate the key does not exist similar to the "containsKey(...)" function in ArduinoJson or fix the jsonExtract function to support passing an empty string when key doesn't exist.