Add NotBlankAfterInstall constraint and use it for system.date:timezone.default

Seems there may be other config that needs updating

Drupal\Tests\system\Functional\Datetime\DrupalDateTimeTest::testDateTimezone
    Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for
    system.date with the following errors: 0 [timezone.default] This value
    should not be null.

Hmm, no relations, setting a parent so it doesn't disappear. Seems rather usefull and quite isolated to implement.

And a quick rebase.

Fun, when looking at the tests, (core/modules/system/tests/src/Functional/Datetime/DrupalDateTimeTest.php:44) it sets an empty timezone as default. But that it not really allowed, or at least, possible through the interface.

So the test starts in a state which is not allowed. That is weird. Looking at the form, it would never happen, and if it was empty it would be the system default. Should that line be set to a default? Should we disable config validation there? Not sure.

Well, this is fun. There is quite a few ways to install config in tests, and a lot of times it installs the config as is default. This means without a value here. While installing only really checks when a install is ran, but not really if a module is being installed.

Not sure how we can do this, i mean, we would like to validate config that is being installed right? So its mostly something during testing, But not sure how to get to the end here.