Fix deferrable mode execution in WasbPrefixSensor

[phanikumv]

WasbPrefixSensor current fails with below error when deferrable flag is set to True. This PR fixes this issue.

Traceback (most recent call last):
  File "/opt/airflow/airflow/providers/microsoft/azure/sensors/wasb.py", line 194, in execute_complete
    raise AirflowException(event["message"])
airflow.exceptions.AirflowException: __aexit__

Post this fix, the sensor executes as expected

[2023-05-19, 11:08:06 UTC] {triggerer_job_runner.py:608} INFO - Trigger example_wasb_sensors/manual__2023-05-19T11:07:51.929984+00:00/wasb_prefix_sensor/-1/1 (ID 1) fired: TriggerEvent<{'status': 'success', 'message': 'Prefix example_az found in container test-container-providers'}>
[2023-05-19, 11:08:09 UTC] {standard_task_runner.py:85} INFO - Job 28: Subtask wasb_prefix_sensor
[2023-05-19, 11:08:09 UTC] {task_command.py:410} INFO - Running <TaskInstance: example_wasb_sensors.wasb_prefix_sensor manual__2023-05-19T11:07:51.929984+00:00 [running]> on host 47367d127faa
[2023-05-19, 11:08:10 UTC] {wasb.py:195} INFO - Prefix example_az found in container test-container-providers

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