title: adding null_value through spring-data-elasticsearch annotation
body: <p>I want to create user index like below using spring-data-elasticsearch-2.1.0. annotation.
  I am not able to find any annotation to add <strong>"null_value": "NULL"</strong>. This is required because our sorting order is failing.</p>
  
  <pre><code>"user": {
      "properties": {
        "firstName": {
          "type": "string"
        },
        "lastName": {
          "type": "string"
        },
        "displayName": {
          "type": "string",
          "analyzer": "word_analyzer",
          "fields": {
            "raw": {
              "type": "string",
              "index": "not_analyzed",
              "null_value": "NULL"
            }
          }
        }
      }
    }
  </code></pre>
  
  <p>Domain class</p>
  
  <pre><code> private String firstName;
   private String lastName;
   @MultiField(
           mainField = @Field(type = FieldType.String, analyzer = "word_analyzer"),
           otherFields = {
                 @InnerField(suffix = "raw", type = FieldType.String, index = FieldIndex.not_analyzed)
                  }
  )
  private String displayName;
  </code></pre>
  
  <p>How to add <strong>"null_value": "NULL"</strong> through spring-data-elasticsearch annotation in InnerField? I do not want to creating index mapping externally.</p>