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

...so 79f52d249fe043e6ec54be6ffa0c71a748920394 "convert sw/source/ui/inc/e*.hxx
from String to OUString" should have converted it to call OUString::getLength,
not OUString::isEmpty.

Change-Id: Ie5633a1ff87094eb22179f14aab4b1508735a747
(cherry picked from commit d0e4d1f01ef44474145469b7203ad3232e36de63)
Reviewed-on: https://gerrit.libreoffice.org/7445
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>