Fix NPE in LoginCredentials.toString

Was seeing NPE if LoginCredentials was constructed with a null password or null privateKey (rather than `Optional.Absent`).

I briefly toyed with the idea of having the field `password` never being null (i.e. in constructor, set to `Optional.absent` if null is passed in), but that would also change behaviour of the public `getOptionalPassword` (and same for `getOptionalPrivateKey`) which are explicitly marked as `@Nullable`.

Therefore this trivial fix seems best + safest for the 1.7.x branch.