title: how to filter null values from map in Dart
body: <p>Following the map, having both key-value pair as dynamic, Write a logic to filter all the null values from Map without us?</p>
  
  <p>Is there any other approach than traversing through the whole map and filtering out the values (Traversing whole map and getting Entry Object and discarding those pairs) ?</p>
  
  <p><strong>I need to remove all values that are null and return map</strong></p>
  
  <pre><code>Map&lt;String, dynamic&gt; toMap() {
   return {
    'firstName': this.firstName,
    'lastName': this.lastName
  };
  </code></pre>