Enable Nullable Reference Types

[gleono]

I mentioned that this could be a good improvement on the code for this runtime in #91

I probably got carried away with some other changes not specific to enabling NRTs. I will explain my rationale for these and ask questions about other minor details in the code.

[gleono]

I forgot to mention that I still cannot run the tests locally in my machine since I can only run openwhisk in standalone mode in a docker container and I don't know how to run the tests against it.

I tested manually by running the container for this runtime and sending the base64 encoded tests to it in the expected JSON text. It seems to work fine that way.

I found some issues while trying to test it in integration with openwhisk in standalone mode. Specially the array input and output cases.