cql3: add handling null value in to_json
cql3: add handling null value in to_json

Previously to_json function would fail with null passed as a parameter.

Fixes #3667

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