Support nullable timestamp columns in time range window functions (#6557

)

* Fix time range window queries with null timestamps:

1. Initial commit. Fix for non-grouped time-ranges with
   null timestamps, in ASC order.
2. Fixed non-grouped time-ranges with null timestampw, in DESC.
3. Fixed grouped time-ranges with null timestamp, in ASC.
4. Fixed grouped time-ranges with null timestamp, in DESC.
5. Fixed grouped time-ranges for ASC (following)
6. Tests for different combinations of timestamp null grouping,
   timestamp ordering, and key grouping.
7. Refactor: Common code gathered to utility function.
8. Code formatting

* Fix time range window queries with null timestamps

Added to CHANGELOG.md.

* Fix time range window queries with null timestamps

1. Non-groupby case: Use null-count to calculate null bounds
2. Groupby case: Use partition_point()

* Fix time range window queries with null timestamps

1. Short circuit eval for all-null/no-null cases.
2. Test cases for all-null/no-null.
3. Code formatting

* Fix time range window queries with null timestamps

1. Fixed angle-brackets for header inclusion.
2. Switched to doxygen-style function documentation
3. Code formatting

* Fix time range window queries with null timestamps

1. Streamline null_bound calculation, with fewer comparisons.
2. Fix botched null_bound calculation cleanup.
3. Remove errant println in tests.

CHANGELOG.md

@@ -24,6 +24,7 @@
 - PR #6504 Update Java bindings version to 0.17-SNAPSHOT
 - PR #6537 Refactor ORC timezone
 - PR #6527 Refactor DeviceColumnViewAccess to avoid JNI returning an array
+- PR #6557 Support nullable timestamp columns in time range window functions
 - PR #6566 Remove reinterpret_cast conversions between pointer types in ORC
 - PR #6544 Remove `fixed_point` precise round
 - PR #6555 Adapt JNI build to libcudf composition of multiple libraries