KPluginMetaData::metaDataFileName: Fix broken check if we have the metaDataFileName value set
With 1b8500d2 we have a d-pointer in more situations than before, consequently checking if the pointer exists can lead to misleading results and the method returning an empty string.
In the long run the method will become obsolete, see https://phabricator.kde.org/T14778#263270, but we should not regress it during KF5.