check for empty string before accessing first element

E.g. fdo#64530-1.xlsx

Change-Id: Iff2d026f6e876ab76c9b89b8200413d6c3a7a0e3
workaround for DBG_UTIL XML correctness checks

The current check will always be hit as the StartXmlElement does not use
a call that puts the element on the stack. So copy the pattern for the
EndXmlElement class.

However we should work on getting rid of these ugly hacks.

Change-Id: Id1141f4afc78a0cae1e4b7accae76e6ae08b77a7