InMemoryClientRegistrationRepository should accept an empty set of registrations

[espenhw]

InMemoryClientRegistrationRepository can be constructed with either a List of ClientRegistrations or a Map of same. However,
passing an empty list is a runtime error while passing an empty map is not.

Commit e554547 changed the assertion in the list case from notNull to notEmpty.

IMO, an empty repository is a perfectly valid use case, so the list constructor should also accept an empty list.

[jzheaux]

@espenhw, thanks for the suggestion; however e554547 brings the var args and list constructors into alignment and both require the list to be non-empty. This is also consistent across the servlet and reactive versions.

Can you better explain your use case and why you need to be able to construct a repository with no values?

[espenhw]

Once again: There is a constructor that takes a Map (https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/oauth2/client/registration/InMemoryClientRegistrationRepository.html#%3Cinit%3E(java.util.Map)). That constructor accepts empty input, while the list- and varargs-taking constructors do not. It should be consistent.

As for why you'd want an empty registry: Imagine you have support for a configurable set of OIDC providers in addition to other authentication sources. Removing all the OIDC-related wiring just because there happens to be no OIDC providers configured isn't feasible.