Use OString::boolean in docxattributeoutput.cxx

Change-Id: I73b0c0ecd3ecf4e159f31e7ad9684c3e70b1ce5e
Reviewed-on: https://gerrit.libreoffice.org/10622
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>