Check for empty tables in zoom poll results (#897)

Co-authored-by: Jason Walker <[email protected]>

parsons/zoom/zoom.py

@@ -151,6 +151,8 @@ def __process_poll_results(self, tbl: Table) -> Table:
         `Returns`:
             Parsons Table
         """
+        if tbl.num_rows == 0:
+            return tbl
 
         # Add surrogate key
         tbl.add_column("poll_taker_id", lambda _: str(uuid.uuid4()))