The merger could return end-of-stream if some (but not all) of the
underlying readers were empty (i.e. not even returning a
`partition_start`). This could happen in places where it was used
(`time_series_sstable_set::create_single_key_sstable_reader`) if we
opened an sstable which did not have the queried partition but passed
all the filters (specifically, the bloom filter returned a false
positive for this sstable).
The commit also extends the random tests for the merger to include empty
readers and adds an explicit test case that catches this bug (in a
limited scope: when we merge a single empty reader).
It also modifies `test_twcs_single_key_reader_filtering` (regression
test for #8432) because the time where the clustering key filter is
invoked changes (some invocations move from the constructor of the
merger to operator()). I checked manually that it still catches the bug
when I reintroduce it.
Fixes#8445.
Closes#8446.
(cherry picked from commit 7ffb0d826b)
c05d8fcef1