[BUG] group_merge_tdigest() always produces one empty cluster regardless of the number of groups when all values are null

[jihoonson]

Describe the bug
This bug was originally reported in NVIDIA/spark-rapids#11367.

When all values are null, group_merge_tdigest() always produces only one empty cluster regardless of the number of groups in the result key column. As a result, the result table cannot be created because of the column size mismatch between the key and the value columns.

Steps/Code to reproduce bug
Repro steps are described in NVIDIA/spark-rapids#11367 (comment).

Expected behavior
group_merge_tdigest() should return the same number of empty clusters as the keys in the result.