wireguard: skip peer config if public key of the peer is not defined

wireguard: skip peer config if public key of the peer is not defined

If a config section of a peer does not have a public key defined, the
whole interface does not start. The following log is shown

daemon.notice netifd: test (21071): Line unrecognized: `PublicKey='
daemon.notice netifd: test (21071): Configuration parsing erro

The command 'wg show' does only show the interface name.

With this change we skip the peer for this interface and emit a log
message. So the other peers get configured.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>