fix(spanner): guard against empty exc.errors in run_in_transaction

When run_in_transaction catches an Aborted exception with an empty
errors list (e.g. manually instantiated Aborted('...')), it crashes
with an uncaught IndexError on exc.errors[0]. Add a guard to fall
back to default_retry_delay when exc.errors is empty.

Closes #17903

Author: abhi-0203

packages/google-cloud-spanner/google/cloud/spanner_v1/session.py

@@ -543,7 +543,9 @@ def run_in_transaction(self, func, *args, **kw):
                 except Aborted as exc:
                     previous_transaction_id = txn._transaction_id
                     delay_seconds = _get_retry_delay(
-                        exc.errors[0], attempts, default_retry_delay=default_retry_delay
+                        exc.errors[0] if exc.errors else None,
+                        attempts,
+                        default_retry_delay=default_retry_delay,
                     )
                     attributes = dict(delay_seconds=delay_seconds, cause=str(exc))
                     attributes.update(span_attributes)
@@ -580,7 +582,9 @@ def run_in_transaction(self, func, *args, **kw):
                 except Aborted as exc:
                     previous_transaction_id = txn._transaction_id
                     delay_seconds = _get_retry_delay(
-                        exc.errors[0], attempts, default_retry_delay=default_retry_delay
+                        exc.errors[0] if exc.errors else None,
+                        attempts,
+                        default_retry_delay=default_retry_delay,
                     )
                     attributes = dict(delay_seconds=delay_seconds)
                     attributes.update(span_attributes)