Enum constraints don't constrain anything

[maxime-rainville]

SilverStripe 4 DBEnum are created as table constraint. Those constraints are created using the following SQL statement.

ADD CONSTRAINT "ClassNamesUpgrade_ClassName_check" check ("ClassName" in ('App\MySite\FooBar', null))"

The only problem is that when you stick null in the list of allowed values, it seems to accept any other values ... which kind of defeats the entire purpose of setting the constraint in the first place.

Note that this is only a problem if you bypass the ORM to set invalid data in your enum column.

There's some related RFC about dropping ENUM support altogether silverstripe/silverstripe-framework#8401 so this might not be worth fixing.

[dnsl48]

Meanwhile I suggest we update the constraint to something like check ("fld" in ('a', 'b') or "fld" is null)

[dnsl48]

After patching this up we'll also need to follow up on #104 (comment) and add a negative test.