title: GSON throws exception parsing empty Date field
body: <p>I'm using GSON to deserialise some JSON.
  The JSON is:</p>
  
  <pre><code>{
      "employee_id": 297,
      "surname": "Maynard",
      "givenname": "Ron",
      "lastlogin": "",
  </code></pre>
  
  <p>...</p>
  
  <p>The Employee Object has a Date field lastlogin:</p>
  
  <pre><code>public class Employee {
  private Integer employee_id;
  
  private String surname;
  
  private String givenname;
  
  private Date lastlogin;
  </code></pre>
  
  <p>The problem I have is that when the lastlogin value isn't populated, it's an empty String in the JSON, so the GSON parser throws:</p>
  
  <pre><code>java.text.ParseException: Unparseable date: ""
  at java.text.DateFormat.parse(DateFormat.java:337)
  at com.google.gson.internal.bind.DateTypeAdapter.deserializeToDate(DateTypeAdapter.java:79)
  at com.google.gson.internal.bind.DateTypeAdapter.read(DateTypeAdapter.java:66)
  </code></pre>
  
  <p>What's the usual way around this?</p>