Fixing SES backend - source (From) must be string.

[satishmittal111]

The Source in send_raw_email must be a String type. mail_from=None is failing the SES backend for airflow alerts.

Currently SES Backend is failing with error -

  File "/Users/username/Library/Python/3.8/lib/python/site-packages/airflow/models/taskinstance.py", line 1852, in email_alert
    send_email(self.task.email, subject, html_content)
  File "/Users/username/Library/Python/3.8/lib/python/site-packages/airflow/utils/email.py", line 52, in send_email
    return backend(
  File "/Users/username/Library/Python/3.8/lib/python/site-packages/airflow/providers/amazon/aws/utils/emailer.py", line 39, in send_email
    hook.send_email(
  File "/Users/username/Library/Python/3.8/lib/python/site-packages/airflow/providers/amazon/aws/hooks/ses.py", line 95, in send_email
    return ses_client.send_raw_email(
  File "/Users/username/Library/Python/3.8/lib/python/site-packages/botocore/client.py", line 357, in _api_call
    return self._make_api_call(operation_name, kwargs)
  ......
  File "/Users/username/Library/Python/3.8/lib/python/site-packages/botocore/validate.py", line 293, in serialize_to_request
    raise ParamValidationError(report=report.generate_report())
botocore.exceptions.ParamValidationError: Parameter validation failed:
Invalid type for parameter Source, value: None, type: <class 'NoneType'>, valid types: <class 'str'>

Refer - https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/ses.html#SES.Client.send_raw_email

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[uranusjr]

Do you need any help on this PR?

[satishmittal111]

@uranusjr I have closed this PR. Created new with right branch name - #16166 Please help for the changes.