When building where clauses it’s not that well documented how to use NOT and NULL .

Starting with a simple query:

    $post = Post::find()->all();

If we wanted to select all posts where the content had something in it

    $post = Post::find()
      ->where(['not',['content'=>null]])
      ->all();

Note the doubles set of square brackets, first before the ‘not’ and then around the parameters.
So, that could also be

    $post = Post::find()
      ->where([ 'not',[ 'content'=>null, 'status' => 1 ]])
      ->all();

Which would equate to content is not null and status not equal 1.