Release notifications raise slack error

[dankolbman]

The Slack notifications for release status changes occasionally break on account of receiving text fields that are empty. This seems to happen primarily on release canceled or failed notifications.

Response from Slack API:

 {'ok': False, 'error': 'invalid_blocks', 'errors': ['failed to match all allowed schemas [json-pointer:/blocks/1/text]', 'must be more than 0 characters [json-pointer:/blocks/1/text/text]'], 'response_metadata': {'messages': ['[ERROR] failed to match all allowed schemas [json-pointer:/blocks/1/text]', '[ERROR] must be more than 0 characters [json-pointer:/blocks/1/text/text]']}}