title: avro maven plugin can not generate code for map schema
body: <p>My avsc file is</p>
  <pre><code>{&quot;type&quot; : &quot;map&quot;,
   &quot;name&quot; : &quot;valueMap&quot;,
   &quot;namespace&quot; : &quot;example.avro&quot;,
   &quot;values&quot;: &quot;long&quot;
  }
  </code></pre>
  <p>but after I used <code>mvn compile</code>, no error reported but no specific class generated.
  when the avsc schema is about <code>enum</code> or <code>record</code>, the avro maven plugin works well.
  How can I generate the related class for map schema?</p>