Fix relations getRelationsForEvent create relation

[hannojg]

The problem

Currently, when calling getRelationsForEvent on a timeline set it will return undefined when no relation event exists. In our app, we have the following logic:

Message component:

const Message = (props) => {
  ...
  useEffect(() => {
    const client = RNMWrapper.getClient();
    const relation = client
      .getRoom(matrixEvent.getRoomId())
      ?.getLiveTimeline()
      .getTimelineSet()
      .getRelationsForEvent(matrixEvent.getId(), "m.moment.seen", "m.reaction");
    relation?.addListener("Relations.add", (e) => {
      // process newly added relation on message
    });
  }, [matrixEvent]);

When a relation is added for matrixEvent we won't know it, as relation is undefined.

The change

I implemented (and wrote tests) that when getRelationsForEvent a new relation will be created if none exists. I think this behaviour is fine because we can then do relation.getRelations() which will return an empty array, which is more descriptive than just returning undefined.

However, I am not sure whether this change would be breaking?

Happy to discuss this change with you ✌️

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix relations getRelationsForEvent create relation (#1717). Contributed by @hannojg.