fix incompatible clustering check

Author: TrevorBergeron

bigframes/session/bq_caching_executor.py

@@ -253,15 +253,14 @@ def _maybe_find_existing_table(
             if spec.if_exists == "fail":
                 raise ValueError(f"Table already exists: {spec.table.__str__()}")
 
-            if (
-                (len(spec.cluster_cols) != 0)
-                and (table.clustering_fields is not None)
-                and (tuple(table.clustering_fields) != spec.cluster_cols)
-            ):
-                raise ValueError(
-                    "Table clustering fields cannot be changed after the table has "
-                    f"been created. Requested clustering fields: {spec.cluster_cols}, existing clustering fields: {table.clustering_fields}"
-                )
+            if len(spec.cluster_cols) != 0:
+                if (table.clustering_fields is None) or (
+                    tuple(table.clustering_fields) != spec.cluster_cols
+                ):
+                    raise ValueError(
+                        "Table clustering fields cannot be changed after the table has "
+                        f"been created. Requested clustering fields: {spec.cluster_cols}, existing clustering fields: {table.clustering_fields}"
+                    )
             return table
         except google.api_core.exceptions.NotFound:
             return None