Bug 31087: Prevent stringification of null in return claims

Bug 31087: Prevent stringification of null in return claims

The return claims table was stringifying 'null'. This patch updates the
code to check for definition so we don't stringify incorrectly

Note: This patch was split out from a follow-up on bug 28854 as we felt
it should be treated separately for backportability.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>