[BUG] Skip Empty Partitions in Bulk Sample Writing (#3600)

Currently, when the bulk sampler writes its samples, it does not check whether the partition is empty.  This causes all of the empty partitions to collide and attempt to write to the same (empty) file at once.  This writing should not occur at all.

This PR adds a check that skips writing if the partition is empty.

This should resolve the nightly test failures.

Authors:
  - Alex Barghi (https://github.com/alexbarghi-nv)

Approvers:
  - Vibhu Jawa (https://github.com/VibhuJawa)
  - Brad Rees (https://github.com/BradReesWork)

URL: #3600

python/cugraph/cugraph/gnn/data_loading/bulk_sampler_io.py

@@ -43,7 +43,7 @@ def _write_samples_to_parquet(
  """
 
  # Required by dask; need to skip dummy partitions.
- if partition_info is None:
+ if partition_info is None or len(results) == 0:
  return
  if partition_info != "sg" and (not isinstance(partition_info, dict)):
  raise ValueError("Invalid value of partition_info")