We need to be careful what data we output to represent "no value"
as it could be mis-interpreted (for example Nan or empty string).

Change-Id: Ie5d85bd58118b0526750f387553dfd7a12b272a4
Reviewed-on: https://gerrit.libreoffice.org/37212Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>