tuple: make fields nullable by default except array/map

Since e1d3fe8a (tuple format: don't
allow null where array/map is expected) tuple fields are non-nullable
by default. It seems strange at least in case we have implicit fields
in front of explicit nullable field. Also it causes incorrect behaviour
in case of using explicitly nullable array/map fields for multikey
index.
Now fields are nullable by default except arrays & maps, as far
as their implicit nullability might break field accessors expectations,
provide confusing error messages and cause incorrect behaviour of
tuple_multikey_count(). In case explicitly nullable array/map fields
are being used for multikey index, clear error message is provided.

Closes #5027