database: make incremental_reader_selector robust vs. full-range partition_range
database: make incremental_reader_selector robust vs. full-range partition_range

incremental_reader_selector assumes the partition_range it receives has a lower
bound, but it was seen in mutation_test that this is not so.

Fix by checking whether the bound exists or not.
Message-Id: <af84eda5456c9aac032efffba32e09db2a470a7b@scylladb.com>

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