Optimization in KDE Base
Do not store default values in QHash<QByteArray, QVariant>
Storing values which are equivalent to default-constructed QVariants
does not make much sense because QHash::value returns the same value
even if the corresponding key is not found in the hash.
This commit reduces Dolphin's memory consumption in large folders by
up to 7.3% (tested a folder with 100,000 files in Details View) and
reduces the time required for loading a folder.