Said method currently emits a partition-end. This method is only called
when the last fragment in the stream is a range tombstone change with a
position after all clustered rows. The problem is that
consume_partition_end() is also called unconditionally, resulting in two
partition-end fragments being emitted. The fix is simple: make this
method a no-op, there is nothing to do there.
Also add two tests: one targeted to this bug and another one testing the
crawling reader with random mutations generated for random schema.
Fixes: #11421Closes#11422
be9d1c4df4