Fixing notice while setting default filters

Fixing error: 
> While setting default filter for column $grid->setDefaultFilters(array('entity.referencedEntity.id' > => array('operator' => 'isNotNull'));
> I got Notice: Undefined index: in APY/DataGridBundle/Grid/Grid.php line 1032
> if (is_bool($value['from'])) {
> $value['from'] = $value['from'] ? '1' : '0';
> }