vinyl: fix bug when tuple not committed to unique nullable index

A unique nullable key definition extended with primary key parts
(cmp_def) assumes that two tuples are equal *without* comparing
primary key fields if all secondary key fields are equal and not
nulls, see tuple_compare_slowpath(). This is a hack required to
ignore the uniqueness constraint for nulls in memtx. The memtx
engine can't use the secondary key definition as is (key_def) for
comparing tuples in the index tree, as it does for a non-nullable
unique index, because this wouldn't allow insertion of any
duplicates, including nulls. It couldn't use cmp_def without this
hack, either, because then conflicting tuples with the same
secondary key fields would always compare as not equal due to
different primary key parts.

For Vinyl, this hack isn't required because it explicitly skips
the uniqueness check if any of the indexed fields are nulls, see
vy_check_is_unique_secondary(). Furthermore, this hack is harmful
because Vinyl relies on the fact that two tuples compare as equal by
cmp_def if and only if *all* key fields (both secondary and primary)
are equal. For example, this is used in the transaction manager,
which overwrites statements equal by cmp_def, see vy_tx_set_entry().

Let's disable this hack by resetting unique_part_count in cmp_def.

Closes #9769

NO_DOC=bug fix

(cherry picked from commit 2e689063)