add phpstan assertions to Collection isEmpty and isNotEmpty

[johanrosenson]

This PR will add @phpstan-assert-if-true / @phpstan-assert-if-false to the Collection methods isEmpty and isNotEmpty.

This will allow phpstan to know if first() will return null or not (when not passing any filter arguments).

Example

/**
 * @var Collection<int, string>
 */
$collection = new Collection(['laravel']);

if ($collection->isNotEmpty()) {
    // before this PR:
    \PHPStan\dumpType($collection->first()); // Dumped type: string|null
    
    // after this PR:
    \PHPStan\dumpType($collection->first()); // Dumped type: string
}

[spawnia]

This is wrong if items in the Collection are null.