It looks like either something changed in the latest v2023.8.x versions,
or when using Biometrics to login, but the MasterPasswordHash is an
optional value during the Approve action.
This PR makes the MasterPasswordHash an optional value which resolves
this issues. Bitwarden works the same way.
I also changed the EncKey to an Option in the database since empty
strings as a default value is not nice in databases, better to use
`null` in these cases.
Fixes#3819
