Don't crash when accessing statistics of empty item repository

To do that, we need to really distinguish between a fully empty
item repository and one that is at least partially initialized.
Most notably, truly empty means we really never have accessed any
buckets and as such various lazy structures are not initialized
yet, such as the m_freeSpaceBuckets.

In real-world usage this change has no impact, as we will still
lazily grow the item repository when we call allocateNextBuckets,
either from `load` (via ItemRepositoryRegistry::registerRepository)
or when we call `index` and `useBucket >= m_buckets.size()`.