title: antd design select placeholder issues
body: <p>I am using antd design in my React app. </p>
  
  <p>Here's a code snippet where I am facing the issues :</p>
  
  <pre><code>&lt;Select
       showSearch
       optionFilterProp = "children"
       placeholder = "Select Company"
       value = "{this.state.company}"
       name = "company"
       onSelect = "{this.handleCompanyChange}"
      &gt;
  </code></pre>
  
  <p>Now it shows the correct value selected if <code>this.state.company</code> is not <code>null</code>. But if <code>this.state.company</code> is <code>empty</code> or <code>null</code>, placeholder doesn't shows up. </p>
  
  <p>How can I solve this issue so that the placeholder appears if <code>value</code> is null?</p>