Fix GithubDriver nextPage deprecation notice with no link header

[alexander-schranz]

Currently getting 2 notices on PHP 8.1 (on preview and snapshot channel) Dont get confused by the version number in the path its come from brew but the tested version is Composer version 2.1.6 2021-08-19 17:11:08 and Composer version 2.1-dev (2.1-dev+5b16d61762579fcd86507292789f1c92432d9e8b) 2021-09-02 15:22:14:

Deprecation Notice: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in phar:///usr/local/Cellar/composer/2.0.8/bin/composer/vendor/symfony/console/Descriptor/TextDescriptor.php:120

This one I think was already fixed by the symfony team.

Deprecation Notice: explode(): Passing null to parameter #2 ($string) of type string is deprecated in phar:///usr/local/Cellar/composer/2.0.8/bin/composer/src/Composer/Repository/Vcs/GitHubDriver.php:579

This check if link is actually returning something if not it is ignored.

Reproducable composer.json:

{
    "name": "sulu/sulu",
    "description": "Test",
    "license": "MIT",
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/derrabus/laminas-code.git"
        }
    ],
    "require-dev": {
        "laminas/laminas-code": "dev-bugfix/return-type-will-change as 4.4.3"
    }
}

[Seldaek]

Thanks