title: Jackson (JSON) throws JsonMappingException when Float is null
body: <p>I am using Spring (with Jackson) and jQuery to pass a form as an object. My pojo includes nullable floats. However, when the fields are empty, Jackson throws an Exeption:</p>
  
  <pre><code>org.codehaus.jackson.map.JsonMappingException: Can not construct instance of java.lang.Float from String value '': not a valid double value
  </code></pre>
  
  <p>What do I have to do in order to allow nulls (or empty values in the form field)?</p>
  
  <p>Thanks
  Er</p>