Allow testing of disabled hooks

What does this MR do and why?

Allows disabled webhooks to be tested manually, fixing #352738 (closed) and unblocking #329849 (closed).

This introduces a force: flag to the webhook execution service, enabling the disabled state of the hook to the overridden, which is required when testing to see if a disabled hook can be re-enabled.

We also ensure that we never return a nil value from the webhook execution itself, since this value is inspected and can result in NPEs.

Screenshots or screen recordings

Showing a history of some failures and then tests which re-enable the webhook:

How to set up and validate locally

See related issue.

MR acceptance checklist

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