Avoid crashes when publishing without WebSocket contexts

[GetThatCookie]

Fixes #1924.

Return false from publish and publishPrepared, and 0 from numSubscribers, when no WebSocketContext has been configured.

This avoids dereferencing a null TopicTree without allocating one or registering loop handlers for HTTP-only apps.

A regression check was added to the existing HTTP-only fuzz target.

Tested by:

• Building all examples
• Running the complete ASan unit test suite
• Running a targeted ASan/UBSan regression test

[uNetworkingAB]

Btw, you come from Node.js right? If so, how did you tackle the NPM change for Git dependencies? You just vendor the binaries?

[GetThatCookie]

Not originally, but Node.js has pretty much become my new world. I’m still using npm 10 because newer npm versions have introduced too many issues in my workflows. So I haven’t had to deal with the Git dependency change yet - I just install the pinned Git dependency normally and don’t vendor the binaries.