render null type fields added and fix render_const

jsonform/gadget_json_generated_form_child.js

@@ -200,12 +200,15 @@
 
   function render_const(schema, json_document) {
     var input = document.createElement("input"),
+      ser_doc = JSON.stringify(json_document),
       ser_const = JSON.stringify(schema.const);
     input.setAttribute('readonly', true);
-    if (deepEqual(json_document, schema.const)) {
+    if (json_document === undefined || deepEqual(json_document, schema.const)) {
+      input.setAttribute('data-origin-value', ser_const);
       input.value = ser_const;
     } else {
-      input.value = JSON.stringify(json_document) + ' ≠ ' + ser_const;
+      input.value = ser_doc + ' ≠ ' + ser_const;
+      input.setAttribute('data-origin-value', ser_doc);
       input.setAttribute('data-const-value', ser_const);
     }
     return input;
@@ -773,13 +776,16 @@
 
     if (json_field.const !== undefined) {
       input = render_const(json_field, default_value);
-      type_changed = true;
     } else if (json_field.enum !== undefined) {
       input = render_enum(json_field, default_value);
       // XXX take in account existing type with enum
       type_changed = false;
     }
 
+    if (!input && type === "null") {
+      input = render_const({const: null}, default_value);
+    }
+
     if (!input && type === "boolean") {
       input = render_boolean(json_field, default_value);
     }