Fix preservation of foreign elements on paragraph merge

This behaviour was working prior to the merge paragraph operation being introduced. A new edge case was introduced related to cleaning up empty spans that caused this to regress.
> For some reason I thought the spans next to each other would be merged into one as well, but that was possibly non-webodf code, at least could not see that happen here, pardon.

That is a future improvement for the character styling logic :crystal_ball:. Unfortunately, the future is not yet here!