Fix command showaccounts

OTAPI_Wrap::GetAccountWallet_Name is used to get account wallet name. The name could be empty so name emptiness should not be used for testing account existence.

Closes #256
@mhamrle we are trying to drop Hungarian notation for code that we touch. Then, the code base will eventually converge to a better naming :)

So please use `name` instead of `strName` etc. in your upcoming patches. Thx!