nixos/network-interfaces: fix implicit dependency on underlying device

the bug causes a hard dependency on an underlying device which might not be
valid for all interfaces, also broke the example for networking.sits. this is
due to calling hasAttr first and checking for null afterwards, the bug was
made more apparent in commit 76a3c30

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -70,15 +70,16 @@ let
         deviceDependency = dev:
           # Use systemd service if we manage device creation, else
           # trust udev when not in a container
-          if (hasAttr dev (filterAttrs (k: v: v.virtual) cfg.interfaces)) ||
+          if (dev == null || dev == "lo") then []
+          else if (hasAttr dev (filterAttrs (k: v: v.virtual) cfg.interfaces)) ||
              (hasAttr dev cfg.bridges) ||
              (hasAttr dev cfg.bonds) ||
              (hasAttr dev cfg.macvlans) ||
              (hasAttr dev cfg.sits) ||
              (hasAttr dev cfg.vlans) ||
              (hasAttr dev cfg.vswitches)
           then [ "${dev}-netdev.service" ]
-          else optional (dev != null && dev != "lo" && !config.boot.isContainer) (subsystemDevice dev);
+          else optional (!config.boot.isContainer) (subsystemDevice dev);
 
         hasDefaultGatewaySet = (cfg.defaultGateway != null && cfg.defaultGateway.address != "")
                             || (cfg.enableIPv6 && cfg.defaultGateway6 != null && cfg.defaultGateway6.address != "");