The "NULL" operator in Expected (old-style conditional operations) doesn't
have any parameters, so we insisted that the AttributeValueList be empty.
However, we forgot to allow it to also be missing - a possibility which
DynamoDB allows.
This patch adds a test to reproduce this case (the test passes on DyanmoDB,
fails on Alternator before this patch, and succeeds after this patch), and
a fix.
Fixes #6816.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20200709161254.618755-1-nyh@scylladb.com>
(cherry picked from commit f549d147ea)