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