fix(link): only warn if "tag" or "event" is used

closes vuejs#3457
Only show deprecation warning if the props "tag" or "event" are actually passed by the consumer (key exists in propsData), not if they have any value at all (they always have a value since a default is specified in the props definition)

src/components/link.js

@@ -132,14 +132,14 @@ export default {
     }
 
     if (process.env.NODE_ENV !== 'production') {
-      if (this.tag && !warnedTagProp) {
+      if ('tag' in this.$options.propsData && !warnedTagProp) {
         warn(
           false,
           `<router-link>'s tag prop is deprecated and has been removed in Vue Router 4. Use the v-slot API to remove this warning: https://next.router.vuejs.org/guide/migration/#removal-of-event-and-tag-props-in-router-link.`
         )
         warnedTagProp = true
       }
-      if (this.event && !warnedEventProp) {
+      if ('event' in this.$options.propsData && !warnedEventProp) {
         warn(
           false,
           `<router-link>'s event prop is deprecated and has been removed in Vue Router 4. Use the v-slot API to remove this warning: https://next.router.vuejs.org/guide/migration/#removal-of-event-and-tag-props-in-router-link.`