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The empty type is used internally to implement CQL sets on top
of multi-cell maps. The map's key (an atomic cell) represents the
set value, and the map's value is discarded. Since it's unneeded
we use an internal "empty" type.
Currently, it is deserialized into a `data_value` object representing
a NULL. Since it's discarded, it really doesn't matter.
However, with the impending change to change lists to allow NULLs,
it does matter:
1. the coordinator sets the 'collections_as_maps' flag for LWT
requests since it wants list indexes (this affects sets too).
2. the replica responds by serializing a set as a map.
3. since we start allow NULL collection values, we now serialize
those NULLs as NULLs.
4. the coordinator deserializes the map, and complains about NULL
values, since those are not supported.
The solution is simple, deserialize the empty value as a non-NULL
object. We create an empty empty_type_representation and add the
scaffolding needed. Serialization and deserialization is already
coded, it was just never called for NULL values (which were serialized
with size 0, in collections, rather than size -1, luckily).
A unit test is added.