Spring BindingReflectionHintsRegistrar#registerReflectionHints can be invoked with a null type

It currently states that the var arg types can be null, which should not happen. The signature should be revisited as well as its callers.

Comment From: snicoll

It turns out that there are various places where we'd actually call the method with null, rather than not calling the registration if no type is available. This probably deserves to be fixed independently of the NullAway change.