Fix issue where passing ['uncheckValue' => null] to BsActiveForm::checkBoxList

When passing ['uncheckValue' => null] to BsActiveForm::checkBoxList, it causes the hidden fields to still be drawn. This is due to the isset() call in BsArray::popValue.

isset($arr[$key]) returns false if $key is not in the array OR if the value is null. Using array_key_exists functions as is expected.