fix(VMenu): check if element still exists before closing parent

[Arinono]

Description

fixes #20248

Instead of using the ! operator, I just added some null checks before using the overlay.value.contentEl.
I don't think it should cause any issue since the issue occurs when the view is completely destroyed (through the router in the reported case), so there is no cleanup to do anyways

Markup:

I used the same reproduction repo I provided in the issue to validate the changes:
https://github.com/Arinono/vuetify-vmenu-bug

<v-menu v-if="open" :close-on-content-click="false">
    <template #activator="{ props }">
      <v-btn v-bind="props">Open</v-btn>
    </template>
    <v-sheet class="pa-4">
      <div class="d-flex">
        <v-menu>
          <template #activator="{ props }">
            <v-btn style="display: block" v-bind="props">Sub menu</v-btn>
          </template>
          <v-sheet class="pa-4"> Click back in the input</v-sheet>
        </v-menu>
        <v-text-field style="width: 400px"></v-text-field>
      </div>
    </v-sheet>
  </v-menu>

  <router-link to="other-route" style="margin-left: 48px"> leave </router-link>

and used this section of the docs to test it