vinyl: never pass NULL key to read iterators

Always use vy_tuple_key_part(key, 0) == NULL
as an indicator of NULL key.

Before this patch, some of the edge case optimizations in
run, mem and txv iterators didn't work, since they compared
input key with NULL, whereas it was set to a special key with zero
parts.

@todo: this is a yet another discrepancy stemming from distinct
tuple formats in memtx and vinyl.