[6.x] fix choosing random elements on <select>

[browner12]

Problem

The browser select() method will currently choose a random option from a <select> input if the second parameter evaluates to null. This normally works fine if it is used like:

$user = (new UserFactory)->make();

$browser->visit('/user/create')
    ->name('name', $user->name)
    ->select('status');

However, it becomes problematic when a second argument is passed that evaluates as null.

$user = (new UserFactory)->make([
    'status' => $faker->optional()->randomElement(['active', 'inactive']),
]);

$browser->visit('/user/create')
    ->name('name', $user->name)
    ->select('status', $user->status);

In this scenario, if $user->status is null, we actually want the <select> to not be set, but instead it will select a random <option>.

It also makes it more difficult to edit a value from a selected option to a null option.

(new UserFactory)->create([
    'status' => 'active',
]);

$user = (new UserFactory)->make([
    'status' => null,
]);

$browser->visit('/user/1/edit')
    ->name('name', $user->name)
    ->select('status', $user->status);

This will again select a random <option>, when we really want the value to be cleared out.

Solution

This PR will switch from checking if the second argument is null, to checking if it was even passed.

This will select a random option:

$browser->select('status');

while this will specifically try to select an option whose value evaluates to an empty string:

$browser->select('status', null);

I go back and forth on if this is a breaking change or a bug fix. If we go by the documentation (https://laravel.com/docs/8.x/dusk#using-forms), this is a bug fix. If you guys deem it as a BC, and can resend to master.