When passed NULL to lok_doc_view_new, use the default path :
$libdir/libreoffice/program as LOK install path

Change-Id: I1e033c407184b29b1509cfb8c416b514591d67ce
Reviewed-on: https://gerrit.libreoffice.org/20476Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>