Protobuf contract must not contain map<string, string> types

In our protobuf contract, "map" type values should never be a native type (if it can be a null value), because it causes a Buffer overrun in JavaScript (and other languages).

https://github.com/Azure/azure-functions-language-worker-protobuf/issues/21

This issue is to track a potential major version change in the protobuf contract to change all map to map (or other nullable type

cc: @pragnagopa

Start by reviewing the protobuf contract and the linked Azure Functions language worker issue to identify every map value that can be null. A complete change would use a nullable value type throughout the contract and account for the required major version change.

Written by the indexing model from the issue text.