Support emitting events from an external process

[dedoussis]

Problem

The validation features of asynction cannot be utilised for events being emitted by an external process.

See the description of the relevant Flask-SocketIO feature: https://flask-socketio.readthedocs.io/en/latest/deployment.html#emitting-from-an-external-process

Proposed solution

When the app is missing from the provided from_spec arguments, then do not register any handlers.