Fix GH-21583: SimpleXML empty element incorrectly casts to true.

[devnexen]

No description provided.

[ndossche]

This changes very long standing behaviour. The current behaviour is here since PHP 5.1.x. PHP 5.0.x had different behaviour, closer to what you propose in the PR but not quite the same.
Since this is changing such old behaviour this can only go into master.
I'm also not so sure whether this should be fixed in the code or if this is a documentation mistake. The new code makes more sense than what it was because now it is similar to a string cast followed by a bool cast.
It raises the question though why the current behaviour is what it is.