Fix scenarios where vue exports an empty part

Happens when there's no code in an element.
Fixes codesandbox#882

Author: CompuIves

packages/app/src/sandbox/eval/transpilers/vue/selector/loader.js

@@ -16,5 +16,6 @@ export default function(code: string, loaderContext: LoaderContext) {
   if (Array.isArray(part)) {
     part = part[query.index];
   }
-  return Promise.resolve({ transpiledCode: part.content });
+
+  return Promise.resolve({ transpiledCode: part ? part.content : '' });
 }