allow empty password

[bdeshi]

It's possible for a kdbx database to be protected by no password.
for example, I maintain a db that's only protected by a keyfile.
but keepmenu was not accepting an empty password. So I've made some minor changes to allow that.

Empty password may be provided by:

1. pressing enter without typing anything at the password prompt.
2. Keeping the password_X field empty in config.

   [database]
   ...
   password_1 =
   ...

[bdeshi]

fixes #108