client/btc: RPC wallet creation can become broken when pass not provided.

[JoeGruffins]

When creating a new btc wallet, and maybe for clones, if you specify an encrypted wallet but forget to add a password, wallet creation can not happen until you restart the client. To reproduce on simnet, create a new client and do not make an internal btc wallet. Then create an external btc wallet with beta config and do not include a password. Wallet settings can no longer be changed with the error: [ERR] WEB: reconfig error: 0 -> btc wallet not found