Fix PostgreSQL enumValues

$column->enumValues is set to array[0 =>''] even if $info['enum_values'] is null.
Gii generates always dropDownList in forms.
Merge PR #3751 branch 'master' of https://github.com/makroxyz/yii2

* 'master' of https://github.com/makroxyz/yii2:
  Fix PostgreSQL enumValues