sstables: reader: Make clustering_ranges_walker work with empty range set
sstables: reader: Make clustering_ranges_walker work with empty range set

Such queries can be issued by counter updates which involve only
static row.

Causes failure in test_query_only_static_row invoked from
sstable_mutation_test. See commit 6572f38, which fixed the problem in
cache reader.

Fixes #2734.

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