fix functional components that return string or nothing (fix #3919)

src/core/vdom/create-component.js

@@ -113,10 +113,12 @@ function createFunctionalComponent (
       slots: () => resolveSlots(children, context)
     }
   )
+  if (vnode instanceof VNode) {
     vnode.functionalContext = context
     if (data.slot) {
       (vnode.data || (vnode.data = {})).slot = data.slot
     }
+  }
   return vnode
 }