db,view: fix checking if partition key is empty
db,view: fix checking if partition key is empty

Previous implementation did not take into account that a column
in a partition key might exist in a mutation, but in a DEAD state
- if it's deleted. There are no regressions for CQL, while for
alternator and its capability of having two regular base columns
in a view key, this additional check must be performed.

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla