fix(setData): allow empty objects to be set

fixes vuejs#1704 

This is a tentative fix to show one possible solution, I can clean it up if you think it makes sense

Author: Haroenv

packages/test-utils/src/recursively-set-data.js

@@ -5,7 +5,7 @@ export function recursivelySetData(vm, target, data) {
     const val = data[key]
     const targetVal = target[key]
 
-    if (isPlainObject(val) && isPlainObject(targetVal)) {
+    if (isPlainObject(val) && isPlainObject(targetVal) && Object.keys(val).length > 0) {
       recursivelySetData(vm, targetVal, val)
     } else {
       vm.$set(target, key, val)