fix type detection for json_document === false

gadget_json_generated_form.js

@@ -638,7 +638,7 @@
     if (getDocumentType(json_field.type) === "string") {
       type = json_field.type;
     } // else json_field.type is array so we use type
-    if (type === undefined && default_value) {
+    if (type === undefined && default_value !== undefined) {
       type = getDocumentType(default_value);
     }