Add Assert.notNull() for AuthenticationProvider additions.

[gmazza]

If we accidentally provide a null authenticationProvider configured via:

            authorizationServerConfigurer
                    .tokenEndpoint(tokenEndpoint -> {
                        tokenEndpoint.accessTokenRequestConverter(new DelegatingAuthenticationConverter(
                            converters))
                            .authenticationProvider(nullAuthProvider)
                        }
                    );

We get an NPE during the authenticationProviders.add(authenticationProvider) statement in Oauth2TokenEndpointConfigurer. The error stack is quite unclear where the NPE is coming from, requiring time-consuming IDE troubleshooting:

Caused by: java.lang.NullPointerException: null
	at org.springframework.security.config.annotation.SecurityConfigurerAdapter$CompositeObjectPostProcessor.postProcess(SecurityConfigurerAdapter.java:117)
	at org.springframework.security.config.annotation.SecurityConfigurerAdapter.postProcess(SecurityConfigurerAdapter.java:79)
	at org.springframework.security.config.annotation.web.configurers.oauth2.server.authorization.AbstractOAuth2Configurer.postProcess(AbstractOAuth2Configurer.java:42)
	at org.springframework.security.config.annotation.web.configurers.oauth2.server.authorization.OAuth2TokenEndpointConfigurer.lambda$init$0(OAuth2TokenEndpointConfigurer.java:129)
	at java.base/java.lang.Iterable.forEach(Iterable.java:75)

This PR adds an Assert.notNull, similar to many places elsewhere in the project, creating a much clearer stack trace:

Caused by: java.lang.IllegalArgumentException: authenticationProvider cannot be null

	at org.springframework.util.Assert.notNull(Assert.java:201)
	at org.springframework.security.config.annotation.web.configurers.oauth2.server.authorization.OAuth2TokenEndpointConfigurer.authenticationProvider(OAuth2TokenEndpointConfigurer.java:91)
	at com.mycompany.myproject.config.SecurityConfig$NewAuthServerConfigurationAdapter.lambda$configure$0(SecurityConfig.java:228)
	at org.springframework.security.config.annotation.web.configurers.oauth2.server.authorization.OAuth2AuthorizationServerConfigurer.tokenEndpoint(OAuth2AuthorizationServerConfigurer.java:162)
	at com.mycompany.myproject.config.SecurityConfig$NewAuthServerConfigurationAdapter.configure(SecurityConfig.java:223)