title: Flask-wtf dynamic select field with an empty option
body: <p>I want to populate a select field based a query search.</p>
  
  <p>But I also want an empty option.</p>
  
  <p>This is my current code</p>
  
  <pre><code> form.state.choices=[(s.id, s.name) for s in State.query.all()]
  </code></pre>
  
  <p>the result is </p>
  
  <pre><code>  &lt;select&gt;
    &lt;option value="CA"&gt;California&lt;/option&gt;
    &lt;option value="FL"&gt;Florida&lt;/option&gt;
    &lt;/select&gt;
  </code></pre>
  
  <p>the desired result is </p>
  
  <pre><code>&lt;select&gt;
  &lt;option value=""&gt;&lt;/option&gt;
  &lt;option value="CA"&gt;California&lt;/option&gt;
  &lt;option value="FL"&gt;Florida&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>It would be cool if is also not valid if the select option is empty.</p>