Both per:
- google authenticator wiki: https://github.com/google/google-authenticator/wiki/Key-Uri-Format#issuer
- yubikey website: https://docs.yubico.com/yesdk/users-manual/application-oath/uri-string-format.html

`issuer` parameter in `otpauth://` URI scheme is optional, even though
it's highly recommended to be present.

This parameter was actually optional in this repo initially, but was
changed to non optional. I'm changing it back to optional.

Co-authored-by: Stefan Seemayer <stefan@seemayer.de>