title: $elemMatch with distinct
body: <p>I have some problems with distinct queries.</p>
  
  <pre><code>db.sessions.distinct("tests.device_serial")
  [
          "",
          "5b34f4bf9854a",
          "5b34f4bf98664",
          "5b34f4bf98712",
          "5b34f4bf9876b",
          "5b34f4bf987c6"
  ]
  </code></pre>
  
  <p>I don't want to get the result with empty strings. I tried to run query:</p>
  
  <pre><code> db.sessions.distinct("tests.device_serial", {"tests.device_serial" : {$ne: ""}})
  [ ]
  </code></pre>
  
  <p>Why I got empty array? Where is my mistake?</p>