Avoid index scan for lower/upper bounds containing NULL keys

[dyemanov]

This PR is a solution for #8278.

If the retrieval is flagged to ignore NULL keys, check whether any used segment of the lower/upper lookup keys contain NULL and if so just skip the index scan returning the empty bitmap.

Stats for the test case inside #8278:

before this PR:

Current memory = 20439712
Delta memory = 50352
Max memory = 20817952
Elapsed time = 36.030 sec
Cpu = 36.020 sec
Buffers = 2048
Reads = 1787
Writes = 4
Fetches = 7560432

after this PR:

Current memory = 18979680
Delta memory = 126736
Max memory = 19058304
Elapsed time = 0.018 sec
Cpu = 0.020 sec
Buffers = 2048
Reads = 1115
Writes = 0
Fetches = 18513