fix(VListItem): subtitle wrong font

[korpusovmax]

fixes #18719

Description

Changed prop condition (props.subtitle) to (props.subtitle !== undefined), as subtitle can be a number (subtitle: [String, Number, Boolean]), and for this reason, if subtitle was 0 (not a string), there was a bug.

Markup:

Code from the issue, now working fine:

<template>
  <v-app>
    <v-container>
      <v-list>
        <v-list-item title="Subtitle is zero" :subtitle="0"></v-list-item>
        <v-list-item title="Subtitle is one" :subtitle="1"></v-list-item>
      </v-list>
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'

  const msg = ref('Hello World!')
</script>

[johnleider]

I'm not sure what the reason was for making subtitle a [String, Number, Boolean], but I think this was a mistake, as everywhere else we use String.

I'd be open to changing this to [Number, String] for all components, VCard/VCardItem/VStepperItem/etc and the corresponding checks to props.subtitle != null.

[johnleider]

Please change the !== null comparisons to != null. Otherwise looks good.