Fixes for #423 (problems with empty password)

Problems fixed:

- null password could potentially be treated as empty string (`HIBitcoinManager.m`)
- change password dialog crashes when current password field is empty (probably https://github.com/hivewallet/hive-osx/issues/292)
- change password dialog allowed setting password to an empty string
- if you changed password to an empty string, you couldn't unlock Hive again or change the password back

@nschum - I'm merging this now, but please do a quick review when you have time, since this is a pretty critical part of the code.