title: Why does mappedResultsQuery() returning an empty Map key?
body: <p>I'm playing around with the build-in Shelf template by adding the <a href="https://pub.dev/packages/postgres" rel="nofollow noreferrer">postgres</a> package.  I modified the <code>_echoHandler</code> function to execute a <code>mappedResultsQuery()</code> like so:</p>
  <pre><code>   Future&lt;Response&gt; _echoHandler(Request request) async {
      var connection = PostgreSQLConnection('localhost', 5432, 'test_db',
          username: 'tester', password: 'testing');
      await connection.open();
  
      List&lt;Map&lt;String, Map&lt;String, dynamic&gt;&gt;&gt; results = await connection
          .mappedResultsQuery('SELECT u.id, u.email, u.name FROM users u LIMIT 1');
  
      for (final row in results) {
          print(row);
      }
  
      final message = params(request, 'message');
      return Response.ok('$message\n');
  }
  </code></pre>
  <p>Which gives me the following output:</p>
  <pre><code>{: {id: 402c70a0-2804-411b-bcd8-f8479ed9f16c, email: john@doe.com, name: John Doe}}
  </code></pre>
  <p>Why does the outer map have an empty key?  Moreover, how is that possible, considering I defined the <code>results</code> List as <code>List&lt;Map&lt;String, Map&lt;String, dynamic&gt;&gt;&gt;</code>?</p>