fix(kmd): handle empty password correctly in getPassword

[PatrickDinh]

fix(kmd): prompt for password twice when the password is an empty string

When using KMD wallet for localnet, it's common that the password is set to an empty string. In this case, the memoised logic failed to detect the existing password.

To fix this:

• the password field is initialised to null
• the memoised logic returns the password if it is not null