[Poll] Improve rendering of poll end message when poll start event isn't available

[mnaturel]

Steps to reproduce

As a user I want to see an informational message every time I receive a m.poll.end event while the m.poll.start event isn’t available. This is especially the case when a user join a room after a poll has been started while the history is not shared for new joiners.

Outcome

What did you expect?

When someone ends a poll and the related m.poll.start event isn’t available, show the localized version of the copy “Ended poll“.

What happened instead?

When someone ends a poll and the related m.poll.start event isn’t available, an empty poll end message is displayed.

Your phone model

No response

Operating system version

No response

Application version and app store

No response

Homeserver

No response

Will you send logs?

No

Are you willing to provide a PR?

Yes