[8.x] Add a way to skip count check but check $callback at the same time for AssertableJson->has()

[s3i7h]

TL;DR

This PR introduces a feature to AssertableJson->has() to skip the count check but check the third argument $callable
(might be a bug?)

Details

Currently the ->has() method of src/Illuminate/Testing/Fluent/AssertableJson.php has a shorthand syntax where you can write:

$assert = AssertableJson::fromArray([
    'bar' => [
        ['key' => 'first'],
        ['key' => 'second'],
    ]
]);

$assert->has('bar', 2, function (AssertableJson $item) {
     $item->where('key', 'first');
})

instead of:

$assert = AssertableJson::fromArray([
    'bar' => [
        ['key' => 'first'],
        ['key' => 'second'],
    ]
]);

$assert
    ->has('bar', 2)
    ->has('bar.0', function (AssertableJson $item) {
        $item->where('key', 'first');
    });

but this syntax doesn't support where you don't want to assert the length but want to check the first item.
such as:

$assert = AssertableJson::fromArray([
    'bar' => [
        ['key' => 'WRONG KEY'],
        ['key' => 'second'],
    ]
]);

// you currently need to explicitly pass 2 as the second argument for this to work
$assert->has('bar', null, function (AssertableJson $item) {
    // never called!!
    $item->where('key', 'first');
})  // and passes!!

This PR removes the check of $length to get into the if clause to call $callback and instead dynamically decide whether to put ->count($length) or not, enabling what's trying to be done above.