Properly handle null FactoryBean instance

Issue: SPR-16250

spring-context/src/test/java/org/springframework/context/annotation/ConfigurationClassPostProcessorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1132,6 +1132,11 @@ public class ConfigurationClassPostProcessorTests {
 		public RepositoryFactoryBean<Object> repoFactoryBean() {
 			return new RepositoryFactoryBean<>();
 		}
+
+		@Bean
+		public FactoryBean<Object> nullFactoryBean() {
+			return null;
+		}
 	}
 
 	@Configuration