title: Exception when search by null value in elasticsearch
body: <p>I am using elasticsearch to search some records in my application.</p>
  
  <pre><code>filtered: {
              query: {
                  query_string: {
                      query: "*#{term}*",
                      fields: ["name"]
                  }
              },
              filter: {
                  bool: {
                      must: [
                          {term: {store_id: store_id}},
                          {match: {dealer: dealer}}
                      ]
                  }
              }
          }
  </code></pre>
  
  <p>When some param is not present and nil value is passed for <code>store_id</code> and <code>dealer</code>. Ii gives following error: <code>{"type":"query_parsing_exception","reason":"No text specified for text query"</code>
  I want if <code>store_id</code> or <code>dealer</code> is nil then it should display all result matching with query. Is it possible in elasticsearch ?</p>