Yii2: Remove "(not set)" in GridView and DetailView for null values

use this:

use Yii;

...

Yii::$app->formatter->nullDisplay = 'N\A';

for kartik\grid\GridView;

'class' => 'kartik\grid\EditableColumn',
'attribute'=>'myAttribute',
'header' => 'myHeader',                                            
'editableOptions' => [                         
    'inputType' => \kartik\editable\Editable::INPUT_TEXT,
    'valueIfNull' => '-',

/**
 * @var string the value to be displayed. If not set, this will default to the attribute value. If the attribute
 * value is null, then this will display the value as set in [[valueIfNull]].
 */
public $displayValue;

Two ways that I know (now):

Formatter

Set nullDisplay of Formatter to something other than null. You can do this in global configuration or for the single GridView or DetailView.

Globally (typically in config/web.php or <application>/config/main.php files):

'components' => [
    ...
    'formatter' => [
        'class' => 'yii\i18n\Formatter',
        'nullDisplay' => '',
    ],
    ...
],

In certain GridView (same with DetailView):

<?= GridView::widget([
    'dataProvider' => $myProvider,
    'formatter' => ['class' => 'yii\i18n\Formatter','nullDisplay' => ''],
    'columns'      => [
        ...
    ],
]); ?>

Set the value

Probably not so elegant. In a certain GridView:

<?= GridView::widget([
    'dataProvider' => $myProvider,
    'columns'      => [
        ...
        [
            'attribute' => 'some_attribute',
            'format'    => 'raw',
            'value'     => function (ModelClass $model) {
                if ($model->some_attribute != null) {
                    return $model->some_attribute; 
              //or: return Html::encode($model->some_attribute)
                } else {
                    return '';
                }
            },
        ],
        ...
    ],
]); ?>

Or in a certain DetailView:

<?= DetailView::widget([
    'model'      => $model,
    'attributes' => [
        ...
        [
            'attribute' => 'some_attribute',
            'value' => $model->some_attribute != null ? $model->some_attribute : '', 
      //or: 'value' => $model->some_attribute != null ? Html::encode($model->some_attribute) : '',
        ],
        ...
    ],
]) ?>

Two hints

If several approaches are used at the same time: setting the value (directly or by function) overrides the formatter configuration of the Grid/DetailView, and this in turn overrides a global formatter configuration.

You can also define something different than an empty string. E.g. if bootstrap is used you may want to use \yii\bootstrap\Html::icon('question-sign') (or '<span class="glyphicon glyphicon-question-sign"></span>') to get a symbol for missing values.