Consistent async file reads: empty files now return no documents

Async text_reader.aread() now returns an empty list ([]) for empty files, aligning behavior with the sync API. This removes special-case handling and simplifies downstream pipelines.

Details

Consistent return types across sync and async code paths
Reduced edge-case logic and clearer semantics for job orchestration
Action required: Update any logic that expects a placeholder empty document

Who this is for: Teams running ingestion pipelines or ETL workflows that need predictable document handling.