Missing foreign key constraints for Node.current_testing_script_set and friends

The 'current_commissioning_script_set', 'current_testing_script_set' and 'current_commissioning_script_set' are declared like this in Node:

    current_testing_script_set = ForeignKey(
        "metadataserver.ScriptSet",
        blank=True,
        null=True,
        on_delete=SET_NULL,
        related_name="+",
    )

But if you look in the database, there's no FOREIGN KEY constraint for that column.

Further more, it seems like the 'on_delete=SET_NULL' also doesn't work properly. Node.start() eventually triggers ScriptResult.create_installation_script_set(), which triggers ScriptResult._clean_old(), which deletes all empty script sets. If one of those are the 'current' scriptset for a node, the Node attribute doesn't get set to None. Instead it leaves a dangling reference to a non-existing script set.

See bug 1996074 for an example where this caused a problem.