In Scylla and Cassandra inserting an empty collection
that is not frozen, is interpreted as inserting a null value.
list_prepare_expression and set_prepare_expression
have an if which handles this behavior, but there
wasn't one in map_prepare_expression.
As a result preparing empty list or set would result in null,
but preparing an empty map wouldn't. This is inconsistent,
it's better to return null in all cases of empty nonfrozen
collections.
Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>
db67ade778