memtx: fix a bug with mvcc and exclude_null option

Before this patch MVCC engine expected that if index_replace
sets `result` to NULL then index_replace sets `successor` to
something (NULL or existing tuple, depending on index type).
That looked fine because by contract `successor` is set when
true insertion was happened.

Unfortunately it was not considered that in case of part with
`exclude_null` option in index the insertion can be silently
skipped and thus `successor` can be not set. The latter access
of it was actually an UB.

Fix it by explicit check of tuple_key_is_excluded and work on
this case correctly.

Note that logically `index_replace` should return a flag whether
the new tuple was filtered (excluded) by key_def. But on the other
hand this flag is required only for mvcc while the function is
already has lots of arguments and it's very cheap to determine
this flag right from memtx_tx, so I decided to make the most
simple patch.

NO_DOC=bugfix