title: Spring Boot String Deserializer for every form field
body: <p>In our web app create and update forms have a size validation. For instance:</p>
  
  <pre><code>@Size(min = 4, max = 20)
  private String mobile;
  </code></pre>
  
  <p>As seen the field is not required. But at the front-end user wants to clear field. Then form validation fails because of length restriction. Incoming data is an empty string instead of null. So minimum length validation restricts the input.</p>
  
  <p>Therefore I start to search a solution to convert empty strings to null values. I found a <code>@InitBinder</code> and <code>StringTrimmerEditor</code> solution but our system uses <code>@ResponseBody</code> approach. So It doesn't fit.</p>
  
  <p>Adding <code>@JsonDeserialize(using = CustomTrimDeserializer.class)</code> annotation or writing a custom setter for every string field is not DRY solution.</p>
  
  <p>I just want to add app wide custom deserializer for String fields.</p>