Validate Parameter in setPostAuthenticationChecks

[big-cir]

Overview

The null check in AbstractUserDetailsReactiveAuthenticationManager#setPostAuthenticationChecks asserts the current field value instead of the method parameter:

public void setPostAuthenticationChecks(UserDetailsChecker postAuthenticationChecks) {
	Assert.notNull(this.postAuthenticationChecks, "postAuthenticationChecks cannot be null");
	this.postAuthenticationChecks = postAuthenticationChecks;
}

Since the field is initialized to a default and can never be null at that point, the assertion always passes and null is silently accepted. The next authenticate(...) call then fails with a raw NullPointerException at .doOnNext(this.postAuthenticationChecks::check) instead of failing fast with a clear message:

manager.setPostAuthenticationChecks(null); // no exception — should throw IllegalArgumentException
manager.authenticate(token).block();       // NullPointerException

All other setters in the same class (setPasswordEncoder, setScheduler, setUserDetailsPasswordService, setMessageSource) validate the parameter. This change fixes the assertion to validate the parameter and adds a regression test.

Related Issue

Closes gh-19276