Fix exception when opening dev tools
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 https://github.com/matrix-org/matrix-react-sdk/pull/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 https://github.com/vector-im/riot-web/issues/6436
