title: flutter sqlite query makes null column into null string
body: <p>In the sqlite table i have null columns. I made a query to get all data from specific column and map its data into <code>List&lt;Map&lt;String, dynamic&gt;&gt;</code>:</p>
  
  <pre><code>List&lt;Map&lt;String, dynamic&gt;&gt; maps = List();
  final db = await database;
  var result = await db.query('tools_table', where: 'LifeCycle &lt;&gt; 2');
  result.forEach((val) {
    maps.add(val);
  });
  </code></pre>
  
  <p>but null column converted into the "null" and when i want to deserialize list of map into object :</p>
  
  <pre><code>ToolsModel.fromJson(Map&lt;String, dynamic&gt; json) {
  ...
      if (json['triggerImagesUrl'] != null) {
        triggerImagesUrl = List&lt;String&gt;();
        var job = jsonDecode(json['triggerImagesUrl']);
        job.forEach((v) {
          triggerImagesUrl.add(v);
        });
      }
  </code></pre>
  
  <p>I got error because json['TriggerUrl'] is nut null, it is "null" and <code>var job = jsonDecode(json['TriggerUrl']);</code> job be a null and job.forEach cause e exception.</p>
  
  <p>Why null column is converted to the "null" and how can i prevent of this action ?</p>