Improve Chat Notification Services Test Settings when Project is empty
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10759 fixed the 500 the users were seeing when the repository was empty by disabling the button in that specific case.
Although this is functional, it does not seem like the correct approach (a user should always be able to test his webhook config).
If we see what was the error being raised we see that it was because we were trying to push to a empty repository.
From all the events I see the only that does not have side effects is the push one (eg: Issue event requires some action taken in a existing issue or an issue being created) and since that fails over a empty reposiotry my proposal would be to create a dummy event that would only send a dummy message through the webhook instead of a push message and therefore not changing anything in the actual project.
\cc @DouweM @dbalexandre