[5.6] Skip null/empty values in SeeInOrder

[3onyc]

Currently passing empty strings to SeeInOrder emits a warning from mb_strpos():

PHP Warning:  mb_strpos(): Empty delimiter in vendor/laravel/framework/src/Illuminate/Foundation/Testing/Constraints/SeeInOrder.php on line 46

As demonstrated in php artisan tinker:

Psy Shell v0.9.4 (PHP 7.2.6 — cli) by Justin Hileman
>>> use Illuminate\Foundation\Testing\Constraints\SeeInOrder;
>>> $const = new SeeInOrder('test');
=> Illuminate\Foundation\Testing\Constraints\SeeInOrder {#2448}
>>> $const->matches(['']);
PHP Warning:  mb_strpos(): Empty delimiter in vendor/laravel/framework/src/Illuminate/Foundation/Testing/Constraints/SeeInOrder.php on line 46

We're currently using assertSeeInOrder to verify pages show model values in a form, but it currently fails on null/empty properties as mentioned above.