GUI: correctly handle 0 as default value

dream/platform/src/fieldset/fieldset.js

@@ -81,7 +81,7 @@
         .push(function () {
           Object.keys(property_list).forEach(function (i) {
             property = property_list[i];
-            value = property._default || "";
+            value = property._default === undefined ? "" : property._default;
             addField(property, value);
           });
         });