don't try to set property mapping if there is no value

Change-Id: I422d651e303553e2ce30895b44b8d96664418c19
Reviewed-on: https://gerrit.libreoffice.org/9850
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>