Display 0 integer values

[arogachev]

0 integer value is treated as "null" and "not set" message appears.
This is because of this empty check:

if (empty($this->valueIfNull)) {
    $this->valueIfNull = '<em>' . Yii::t('kveditable', '(not set)') . '</em>';
}

The workaround I used is setting 'valueIfNull' option to some html string, for example like this: <span>0</span>. As i remember in framework itself the value is treated as "not set" only if it's null.
Does not seem like a big problem, but I think it's better to display 'not set' only if value is null by default.