Filter out headers with empty values for Node.js

[mhoeger]

Note - we probably don't want to filter out for everything because this behavior is only buggy for Node.js. Correct approach is through capabilities and either explicitly opting out of those or introducing a new headers property that is typed correctly.

This issue exists because of the way we implemented headers in our protobuf schema (as a map<string, string>) and the "map" type doesn't accept empty values for runtimes like node. Causes buffer overrun and corrupts other parts of the message :(

Azure/azure-functions-nodejs-worker#142 and Azure/azure-functions-language-worker-protobuf#21