In Ifb0fbaa29170f79666078ff1e954f8e52f279a3f we unintentionally (as far as I can see) unconditionally added an element #mw-html-debug-log even when there is no debugging information.
@Krinkle flagged this during a code review, and I think the element should not be outputted when getHTMLDebugLog is empty
SkinMustache::tailElement should omit the element when it encounters an empty string.
Thoughts?