net/wireguard: Fixed array_merge error for removed peers

[jkellerer]

This PR fixes the following error, triggered when opening the dashboard (with the Wireguard widget and a configuration with dead peers in servers):

[04-Nov-2022 13:53:07 Europe/Berlin] TypeError: array_merge(): Argument #1 must be of type array, null given in /usr/local/opnsense/mvc/app/controllers/OPNsense/Wireguard/Api/GeneralController.php:93
Stack trace:
#0 /usr/local/opnsense/mvc/app/controllers/OPNsense/Wireguard/Api/GeneralController.php(93): array_merge(NULL, Array)
#1 [internal function]: OPNsense\Wireguard\Api\GeneralController->getStatusAction()
#2 [internal function]: Phalcon\Dispatcher\AbstractDispatcher->callActionMethod(Object(OPNsense\Wireguard\Api\GeneralController), 'getStatusAction', Array)
#3 [internal function]: Phalcon\Dispatcher\AbstractDispatcher->dispatch()
#4 /usr/local/opnsense/www/api.php(24): Phalcon\Mvc\Application->handle('/api/wireguard/...')
#5 {main}

(Caused by the fact that $server->peers may contain peers that no longer exist)

[fichtner]

CC @patschi

[fichtner]

Looks like this was confirmed by some people already. Thanks for the patch, merged!