fix: Avoid pushing empty messages

[juliusknorr]

Found in sentry
Fixes a regular js console error when using notify_push where the awareness message may be pushed as empty string.

We avoid this filtering on the backend before we emit the messages over notify_push, but @max-nextcloud do you happen to know if seeing those empty awareness messages being pushed is expected?

Without this PR parsing the empty data fails in

text/src/services/y-websocket.js

Line 167 in ed57736

const encoder = readMessage(provider, new Uint8Array(event.data), true)