title: VueJS select placeholder does not display with v-model not empty
body: <p>here my code :</p>
  
  <pre><code>&lt;b-field style="display:inline-block;width:calc(90% / 4);" label="Par Filiale"&gt;
          &lt;b-select placeholder="Choix de la filiale" v-model="searchFiliale"&gt;
              &lt;option :value="null" disabled&gt;Sélectionner une filiale&lt;/option&gt;
              &lt;option :value="''"&gt;&lt;/option&gt;
              &lt;option
                  v-for="filiale in listServicesPartenairesFiliales.filiales"
                  :value="filiale.name"
                  :key="filiale.id"&gt;
                  {{ filiale.name }}
              &lt;/option&gt;
          &lt;/b-select&gt;
      &lt;/b-field&gt;
  </code></pre>
  
  <p>And the datas :</p>
  
  <pre><code>data() {
          return {
              searchFiliale: this.$root.getParamUrl('filiale') || '',
          }
      },
  </code></pre>
  
  <p>The problem is that the placeholder of b-select does not display because <code>searchFiliale</code> is not empty or null.
  Do you have an alternative to keep <code>searchFiliale</code> in <code>v-model</code> like here but with the good placeholder ?
  Thank you !</p>