"map" types to not contain empty values for node

[mhoeger]

Null or empty values on protobuf map types cause buffer overruns on Node.js: Azure/azure-functions-language-worker-protobuf#21

The ultimate goal is to get rid of any map types that allow for empty values (ex: string): #4482

For now, we may need to implement host changes that filter out empty-valued pairs. This is true for http headers as well as environment variables (which will cause apps with empty-valued environment variables to fail when we implement placeholders).

cc: @pragnagopa and @fabiocav

[pragnagopa]

Note: Do not have to specialize this fix for node worker as the guidance for handling reload requests on the worker is to clear environment variables and replace

[pragnagopa]

Verfied Environment.GetEnvironmentVariables does not include empty or null values.