Initialize all fields in Partition

In addition to !29 (merged)

Partition class lacks initialization for some fields what causes uninitialized data usage. My previous MR fixed this only for new partitions created from KDE Partition Manager via the method. Other applications (e.g. Calamares) might use Partition ctor directly.

There might be some similar oversights in the codebase but I've briefly looked and haven't found any.