Stop requesting null next replies from the server

A recent change (47e007e08f) introduced a
regression where we failed to check whether a reply thread has a next reply.
This meant that we would end up sending `/context/undefined` requests to the
server for every reply thread on every room view.

Fixes https://github.com/vector-im/element-web/issues/17563
Regressed by https://github.com/matrix-org/matrix-react-sdk/pull/6079

src/components/views/elements/ReplyThread.js

@@ -297,6 +297,7 @@ export default class ReplyThread extends React.Component {
     }
 
     async getEvent(eventId) {
+        if (!eventId) return null;
         const event = this.room.findEventById(eventId);
         if (event) return event;