FIX Don't pass null as LeftAndMain error message.

[GuySartorelli]

Fixes https://github.com/silverstripe/silverstripe-security-extensions/runs/7602515088?check_suite_focus=true

SilverStripe\SecurityExtensions\Tests\Control\SudoModeControllerTest::testActivateFailsWithGetRequest
TypeError: SilverStripe\Admin\LeftAndMain::setHttpErrorMessage(): Argument #1 ($message) must be of type string, null given, called in /home/runner/work/silverstripe-security-extensions/silverstripe-security-extensions/vendor/silverstripe/admin/code/AdminErrorExtension.php on line 21

We shouldn't be trying to pass null where we've declared it should be a string.
We have to accept null into this hook anyway as sometimes that's actually passed in - so it still makes sense for that to be the default here. But we shouldn't pass it through.

[GuySartorelli]

Failing behat tests are unrelated - fixed in #1336