Use idiomatic AssertJ assertions for true, false, and null

spring-context-support/src/test/java/org/springframework/cache/caffeine/CaffeineCacheTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -106,7 +106,7 @@ class CaffeineCacheTests extends AbstractValueAdaptingCacheTests<CaffeineCache>
 		Cache.ValueWrapper wrapper = cache.putIfAbsent(key, "anotherValue");
 		// A value is set but is 'null'
 		assertThat(wrapper).isNotNull();
-		assertThat(wrapper.get()).isEqualTo(null);
+		assertThat(wrapper.get()).isNull();
 		// not changed
 		assertThat(cache.get(key).get()).isEqualTo(value);
 	}