String::Len was used in a non-bool context here

...and apparently erroneously got rewritten as a call to OUString::isEmpty
instead of OUString::getLength in the conflict resolution of
7ec7a9a68b4a9c5600840a34434fc800cda3d815 "Fix #120224# Mapping the text rotation
in Writer table cell with MSO."

Change-Id: If02687437848917f64b8f92f323979ccb51aaad2
(cherry picked from commit 60af010a98540b4785353b27ccbb545ef72e7791)
Reviewed-on: https://gerrit.libreoffice.org/7451
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>