vinyl: do not fill secondary tuples with nulls when decoded

In contrast to a primary index, which stores full tuples, secondary
indexes only store extended (secondary + primary) keys on disk. To make
them look like tuples, we fill missing fields with nulls (aka tuple
surrogate). This isn't going to work nicely with multikey indexes
though: how would you make a surrogate array from a key? We could
special-case multikey index handling, but that would look cumbersome.
So this patch removes nulls from secondary tuples restored from disk
altogether. To achieve that, it's enough to use key_format for them -
then the comparators will detect that it's actually a key, not a tuple
and use the appropriate primitive.