title: Angular Formly options value causes Placeholder bug
body: <p>My options dropdown has a boolean value in this Formly field. Whenever the value is false, this causes the placeholder and the options label to be overlayed over each other.</p>
  <p>I don't have this issues with any of my other formly items, just ones with the value of false. Is there a solution for this?</p>
  <p>Edit: I may be having trouble uploading photos, but I found another example of this error here: <a href="https://stackblitz.com/edit/angular-formly-demo" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-formly-demo</a>
  The only difference is I'm not using Material</p>
  <p><a href="https://i.stack.imgur.com/WcHkJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WcHkJ.png" alt="formly-true-option" /></a>
  <a href="https://i.stack.imgur.com/rttTD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rttTD.png" alt="formly-false-option" /></a></p>
  <pre><code>{
                  className: 'dropdown',
                  key: 'emergencyActionPlan',
                  id: 'emergencyActionPlan',
                  type: 'dropdown',
                  templateOptions: {
                    placeholder: 'Emergency Action Plan',
                    required: true,
                    options: [
                      { value: true, label: 'Yes' },
                      { value: false, label: 'No' }
                    ],
                    labelProp: 'label',
                    valueProp: 'value'
                  }
  }
  </code></pre>