wallet2 fails to save to disk if first created in-memory
If wallet2 is first created in-memory (i.e. with empty path `""`), then it will fail to save to disk, due to incorrect assumptions that the other files have already been created, based on this comment and the following logic: https://github.com/monero-project/monero/blob/master/src/wallet/wallet2.cpp#L6238

This issue requests fixing wallet2 to correctly save to disk after being created in-memory with empty path.