server: differentiate between missing and undefined doc in default decision (#5420)

Since "document missing or undefined" caused confusion, we can be
more helpful and report whether the default decision document is
missing, or whether it's there but undefined. I would probably have
preferred for undefined to just result in an empty/undefined result,
but that'd be quite a breaking change at this point in time, so
hopefully this can at least make things a little less convoluted.

Fixes #5344

Signed-off-by: Anders Eknert <anders@eknert.com>