title: Empty string in Elasticsearch date field?
body: <p>I have a date field setup in my mapping like this:</p>
  
  <pre><code>"groupsAssignedDate" : {
      "type" : "date",
      "format" : "MM-dd-YYYY"
  }
  </code></pre>
  
  <p>In my app, the field groupsAssignedDate is set to the empty string initially. When a group is assigned, a unix timestamp is generated and stored in the field. I'm trying to use elasticsearch's bulk update function to bring in a bunch of documents with a groupsAssignedDate of "" because no groups have been assigned yet. Elasticsearch won't index the docs though. This is in the log file:</p>
  
  <pre><code>Failed to parse [groupsAssignedDate]
  ...
  failed to parse date field [], tried both date format [MM-dd-YYYY], and timestamp number
  </code></pre>
  
  <p>It appears elasticsearch supports the JSON null value...do I need to switch my docs from using "" to null or is there some way I can support using the empty string instead?</p>