idmap followup: avoid false-negatives through falsy values in collision check

idmap followup: avoid false-negatives through falsy values in collision check

By using "exists" when checking if a hash entry is set, else things
like "0" could get accepted by mistake.

Also cleanup the code a little, like dropping the "PVE::JSONSchema::"
prefix, this is now in that module after all.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>