title: elasticsearch mapping of geo_point with null_value
body: <p>I ran into an issue related to elasticsearch's <strong>geo_point</strong> while importing from a DB.
  My mapping works great if value for <strong>lon</strong> or <strong>lat</strong> field contains real values.</p>
  
  <p>Now it can happen that some rows contain <strong>empty values</strong> for <strong>lon/lat</strong> and if I try to import these I get a <strong>ElasticsearchParseException[latitude must be a number]</strong> error which totaly seems legit.</p>
  
  <p>So I want to set these fields to <strong>0,0 or NULL,NULL</strong> but this is where I get stuck.</p>
  
  <p>I tried to add <strong>'null_value' => '0'</strong> to the mapping but that doesn't seem to work.</p>
  
  <p>This is how the mapping looks like:</p>
  
  <pre><code>'_source' =&gt; array(
              'enabled' =&gt; true
          ),
          'properties' =&gt; array(
              'coordinates' =&gt; array(
                  'type' =&gt; 'geo_point',
                  'null_value' =&gt; '0'
              )
          )
  </code></pre>
  
  <p>Anyone aware of this? </p>
  
  <p>(I am using the PHP Elasticsearch Client.)</p>