message: Fixes [Vue] placeholder not appearing in dropdown question
diff: <template>
  <div :class="css.dropdown.root">
  <select v-if="!question.isReadOnly" :id="question.inputId" v-model="question.value" :class="css.dropdown.control">
  - <option value="">{{question.optionsCaption}}</option>
  + <option value=undefined>{{question.optionsCaption}}</option>
  <option v-for="(item, index) in question.visibleChoices" :value="item.value">{{item.text}}</option>
  </select>
  <div v-else :text="question.value" :class="css.dropdown.control"></div>
  