fix: don't generate app for host if no providers were used

makeApps.nix

···
     l.mergeAttrsList (
       l.mapAttrsToList
       (
-        hostname: host: {
+        hostname: host:
+        if l.hasAttr "providers" host.config
+        then {
           "deploy-${hostname}-resources" = {
             type = "app";
             program = toString (mkApp host);
           };
         }
+        else {}
       )
       self.nixosConfigurations
     )