Fix problem when inserting a new headline

Fix problem when inserting a new headline

Ryan Thompson writes:

> I have found a bug. When the point is at the end of an empty headline
> and you press M-RET (or C-RET) to make a new headline, it deletes all
> the whitespace at the end of the empty headline first, which causes
> the headline to break. I'm not sure if the correct behavior is to
> leave an empty headline, or maybe just do nothing and leave the point
> at the end of the empty headline without creating a new one, but the
> correct thing is definitely *not* to break the headline.