Fix EZP-23544: UrlStorage::getFieldData logs an error if URL with an empty ID is not found

JIRA issue: https://jira.ez.no/browse/EZP-23544

The logger in `UrlStorage::getFieldData` should only log an error where there's an actual ID stored in the database, and not for ID which is `0` or `null`, which is the default for an `ezurl` field type which has no content.