Assignee_id must be nullable

[tanji]

A primary IP can be unassigned and therefore assignee_id will be null, which led to an exception.

<b>Fatal error</b>:  Uncaught TypeError: Argument 12 passed to LKDev\HetznerCloud\Models\PrimaryIps\PrimaryIp::__construct() must be of the type integer, null given, called in /opt/ops-ip-lister/vendor/lkdevelopment/hetzner-cloud-php-sdk/src/Models/PrimaryIps/PrimaryIp.php on line 166 and defined in /opt/ops-ip-lister/vendor/lkdevelopment/hetzner-cloud-php-sdk/src/Models/PrimaryIps/PrimaryIp.php:96
Stack trace:
#0 /opt/ops-ip-lister/vendor/lkdevelopment/hetzner-cloud-php-sdk/src/Models/PrimaryIps/PrimaryIp.php(166): LKDev\HetznerCloud\Models\PrimaryIps\PrimaryIp-&gt;__construct(0000, 'base-image', '2023-12-23T02:0...', '1.1.1.1', 'ipv4', Array, false, Object(LKDev\HetznerCloud\Models\Protection), Array, Object(LKDev\HetznerCloud\Models\Datacenters\Datacenter), 'server', NULL, false)
#1 /opt/ops-ip-lister/vendor/lkdevelopment/hetzner-cloud-php-sdk/src/Models/PrimaryIps/PrimaryIps.php(163): LKDev\HetznerCloud\Models\PrimaryIps\PrimaryIp::parse(Object(stdClass))
#2 [internal function]: LKDev\HetznerCloud\Models\PrimaryIps\Prim in <b>/opt/ops-ip-lister/vendor/lkdevelopment/hetzner-cloud-php-sdk/src/Models/PrimaryIps/PrimaryIp.php</b> on line <b>96</b><br />