fdo#75397: Return an empty string for empty element.

This is the behavior as of 3.6.  It had changed in 4.0 and onward by
accident, and caused the bug as reported in fdo#75397.

Change-Id: Id96fea354604b3c13cbbf2d9a73223b7725c7d66
(cherry picked from commit 5fab47ddbe332a150fb2005e941a2c19bd38ce7f)
Reviewed-on: https://gerrit.libreoffice.org/9187
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>