feat: Make custom on_exit_notify task more flexible

[afonsobspinto]

Closes https://metacell.atlassian.net/browse/CH-188

Implemented solution

• Checks if queue and payload are present in the on_exit_notify param before trying to retrieve them.

How to test this PR

Added a couple more new unit tests for the different on exit notify scenarios f.e:

    on_exit_notify = {
        'image': 'my-custom-exit-handler'
    }

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Test coverage (select one):

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Documentation (select one):

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[filippomc]

LGTM