Fix exception when opening dev tools

[dbkr]

Apparently draft sometimes gets unhappy when you try to move to the
end of an empty state, so only move to the end if we actually
created a non-empty state.

This will be irrelevant once #1890
is merged, but I have the fix now, and this is probably otherwise
going to annoy me now I'm doing things where devtools is very useful.

Fixes element-hq/element-web#6436

[lukebarnard1]

This will inevitably conflict with #1890 so I'm not sure if it's worth merging it. The unknown is how quickly we can get #1890 reviewed and merged, and this will impact users on /develop between merging this and merging #1890. In conclusion shrugs.

[dbkr]

Given it should be trivial to resolve the conflict, I'm going to merge it.

[dbkr]

Looks like this causes everything to break, predictably enough. Sigh. Let's burn draft with fire.