If an indexed field expects array/map, it shouldn't be allowed to insert
null instead, because this might break expectations of field accessors.
For unikey indexes inserting null instead of array/map works fine though
somewhat confusing: for a non-nullable field you get a wrong error
message ("field is missing" instead of "array/map expected, got nil");
for a nullable field, this silently works, just looks weird as there's a
clear type mismatch here. However, for a multikey field you get a crash
as tuple_multikey_count() doesn't expect to see null where an array
should be according to the format:

  tuple_raw_multikey_count: Assertion `mp_typeof(*array_raw) == MP_ARRAY' failed.

This issue exists, because we assume all fields are nullable by default
for some reason. Fix that and add some tests.

Note, you can still omit nullable fields, e.g. if field "[2].a[1]" is
nullable you may insert tuple [1, {a = {}}] or [1, {b = 1}] or even [1],
you just can't pass box.NULL instead of an array/map.