An empty partition/clustering key pair is a valid state of the
query paging state. Unfortunately, recent attempts at debugging
a flaky test resulted in introducing an assertion which breaks
when trying to generate a key from such a pair.
In order to keep the assertion (since it still makes sense in its
scope), but at the same time translate empty keys properly,
empty keys are now explicitly processed at the beginning of the
function.
This behaviour was 100% reproducible in a secondary index dtest below.
Fixes#6134
Refs #5856
Tests: unit(dev),
dtest(TestSecondaryIndexes.test_truncate_base)
(cherry picked from commit 45751ee24f)
53f47d4e67