Restore lenient null return value for ConditionContext.getBeanFactory()

Includes nullable return value for getClassLoader() with corresponding notes in applicable javadoc.

Issue: SPR-16866

(cherry picked from commit 46a89d9)

spring-core/src/main/java/org/springframework/core/io/ResourceLoader.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.
@@ -72,7 +72,9 @@ public interface ResourceLoader {
 	 * in a uniform manner with the ResourceLoader, rather than relying
 	 * on the thread context ClassLoader.
 	 * @return the ClassLoader
+	 * (only {@code null} if even the system ClassLoader isn't accessible)
 	 * @see org.springframework.util.ClassUtils#getDefaultClassLoader()
+	 * @see org.springframework.util.ClassUtils#forName(String, ClassLoader)
 	 */
 	@Nullable
 	ClassLoader getClassLoader();