Implement JCacheCache#putIfAbsent as atomic operation

This commit modifies putIfAbsent to use an EntryProcessor that
guarantees that the operation is atomic.

Closes gh-21591

spring-context-support/src/test/java/org/springframework/cache/jcache/JCacheEhCacheApiTests.java

@@ -24,9 +24,12 @@ import javax.cache.spi.CachingProvider;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.context.testfixture.cache.AbstractValueAdaptingCacheTests;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * @author Stephane Nicoll
  */
@@ -79,4 +82,22 @@ public class JCacheEhCacheApiTests extends AbstractValueAdaptingCacheTests<JCach
 		return this.nativeCache;
 	}
 
+	@Test
+	void testPutIfAbsentNullValue() {
+		JCacheCache cache = getCache(true);
+
+		String key = createRandomKey();
+		String value = null;
+
+		assertThat(cache.get(key)).isNull();
+		assertThat(cache.putIfAbsent(key, value)).isNull();
+		assertThat(cache.get(key).get()).isEqualTo(value);
+		org.springframework.cache.Cache.ValueWrapper wrapper = cache.putIfAbsent(key, "anotherValue");
+		// A value is set but is 'null'
+		assertThat(wrapper).isNotNull();
+		assertThat(wrapper.get()).isNull();
+		// not changed
+		assertThat(cache.get(key).get()).isEqualTo(value);
+	}
+
 }