large_data_handler: Prepare for stopped qctx
large_data_handler: Prepare for stopped qctx

All the large data handler methods rely on global qctx thing to
write down its notes. This creates circular dependency:

query processor -> database -> large_data_handler -> qctx -> qp

In scylla this is not a technical problem, neither qctx nor the
query processor are stopped. It is a problem in cql_test_env
that stops everything, including resetting qctx to null. To avoid
tests stepping on nullptr qctx add the explicit check.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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