Locations makes more sense to be optional than mandatory

[josep-fl]

Hi there, not really a bug, but a feature request. At the moment, locations is mandatory. That contrasts with Checkly's API where it is optional. Also when creating a check in a group from Terraform, location is mandatory but does nothing, as locations is controlled by group.

All in all, I think it makes sense to make locations optional, and somehow take some location by default.