nixos/test-driver: fix corrupt hosts files

primaryIPAddress is defined using lib.optionalString.  If it is the
empty string (because there are no assigned IP addresses), and the
machine has a hostname, we don't want to generate an invalid hosts
file with that empty string as the first (IP address) field.

nixos/lib/testing/network.nix

@@ -93,7 +93,7 @@ let
               # We also want to test router protocols that enable connections
               # between nodes even if they don't share a VLAN, so we include
               # the primary IPs of all machines in the hosts file.
-              primaryIPs = [
+              primaryIPs = lib.remove "" [
                 remoteConfig.networking.primaryIPAddress
                 remoteConfig.networking.primaryIPv6Address
               ];