Subscribing to events using CoAP returns `undefined`

As discovered by @relu91, there is currently a bug in the CoAP binding implementation that leads to servers returning `undefined` when the initial observe GET request is received. The issue is being caused by this line, leading to the request being responded to before the event handler passes a value:

https://github.com/eclipse-thingweb/node-wot/blob/450ebd97527bb5d31acdfc0f56602da575c09a7d/packages/binding-coap/src/coap-server.ts#L887