lib/jsonschema: render defaults for submodule options

this relaxes the constraint that options of type `submodule` are always
required, and will render benign default values.

lib/jsonschema/test_parseOption.nix

@@ -324,6 +324,7 @@ in
             };
           };
         };
+        default = { };
       };
     };
 
@@ -354,6 +355,7 @@ in
             };
           };
         };
+        default = { };
         required = [ "opt" ];
       };
     };