`rbw edit` inserts empty history entries if there was no previous password

[simias]

It's fairly simple to reproduce:

Using the desktop app create a new entry without password.

Sync rbw and attempt to edit the password using rbw edit on the entry.

You can now see that in the desktop app you have an history entry without password:

To make matters worse, rbw now errors out when attempting to access the vault because it can't deal with empty passwords (see #50).