Bug 20886: Do not cast undef to 0 (TO_JSON)

Bug 20886: Do not cast undef to 0 (TO_JSON)

We should not cast undefined values to 0.

Signed-off-by: Andrew Isherwood <andrew.isherwood@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
This was a bug I introduced. I think the fix is right, and if something
gets broken for this, it means it was making the wrong assumptions on
the data.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>