Graceful handling of missing or empty event.unsigned object.

[jevolk]

Otherwise an exception is thrown.

[jevolk]

Hello, I would just like to note that the Matrix does not place prev_content under unsigned in all places where it's used (except https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-notifications).

This might be a greater issue with the spec, and there may be workarounds in this library I haven't seen yet, but at a glance I think this may be problematic with Construct.

[tulir]

I think I have prev_content moving somewhere, so that part might work fine already