title: how to deal with null values in an elastic search field_value_factor
body: <p>How do I deal with a null value on a property that I want to do a field_value_factor for?  I want to weight by popularity but some records have a null value for that property.  Do I have to set a minimum value of 10 on that property in that data itself?  It seems kind of kludgy that way.</p>
  
  <pre><code>{
    "query": {
        "function_score": { 
          "query":{
            "multi_match" : {
              "query" : "new girl",
              "fields" : [ "title^1.2", "name"] 
            }
          },
          "field_value_factor": {
            "field":"popularity",
            "modifier":"log1p"
          },
          "boost_mode":"multiply"
  
        }
    }
  }
  </code></pre>