Skip Dataflow ThreadPoolExecutor on empty preprocessing spec

Same root cause as the enumerate fix: ThreadPoolExecutor(max_workers=0)
raises ValueError. When both node_ref_to_preprocessing_spec and
edge_ref_to_preprocessing_spec are empty, num_dataflow_jobs is 0 and
the executor blows up in __init__. Early-return an empty
PreprocessedMetadataReferences in that case — no Dataflow work to do.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kmontemayor

gigl/src/data_preprocessor/data_preprocessor.py

@@ -352,6 +352,12 @@ def __build_data_reference_str(references: Iterable[DataReference]) -> str:
             edge_ref_to_preprocessing_spec
         )
 
+        if num_dataflow_jobs == 0:
+            logger.info("No data references to preprocess; skipping Dataflow.")
+            return PreprocessedMetadataReferences(
+                node_data=node_refs_and_results, edge_data=edge_refs_and_results
+            )
+
         with concurrent.futures.ThreadPoolExecutor(
             max_workers=num_dataflow_jobs
         ) as executor: