Allow wallets without passwords

[lyoshenka]

Long-running services should not have to enter a password every time they want to sign a transaction. Encouraging passwords is great and nudging ppl to use them is great too, but it should not be a requirement.

[roylee17]

v0.13.111 merged a few updates which work for this use case.

• During wallet creation, a default passphrase, "passphrase", would be used to protect the seed unless the user overrides it with -p new_passphrase.

• At wallet startup, it unlocks itself with the default "passphrase" or a user-provided one with -p.

If the user doesn't override the default passphrase, the experience will feel like no passphrase at all.
Give it a shot and see if this match the expectation.