fix(router): empty string to represent real empty "to"

[yuwu9145]

fixes #19549

• Empty string is the one to represent real empty "to" that makes RouterLink.useLink happy demo
• to={} in RouterLink.useLink is looking for url "/" demo

Description

Markup:

<template>
  <v-app>
    <v-container>
      <v-list nav>
        <v-list-item to="/">
          home
        </v-list-item>
        <v-list-item to="/foo">
          foo
        </v-list-item>
        <v-list-item :to="to">
          bar
        </v-list-item>
      </v-list>
      <v-btn @click="showTo = !showTo">toggle To</v-btn>
      <hr class="my-4" />
      <h1>
        <router-view></router-view>
      </h1>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data () {
      return {
        showTo: false
      }
    },
    computed: {
      to() {
        if (this.showTo) return '/bar'

        return ''
      }
    }
  }
</script>