wallet JSON update for non-deterministic wallet data

wallet2::store_keys() and wallet2::load_keys() should only use the JSON
attribute "seed_language" when applicable. That is only for
deterministic wallets.

 - store_keys()  don't add JSON attribute "seed_language" if
   seed_language is empty

 - load_keys()  don't call set_seed_language if JSON attribute
   "seed_language" not present