DATAREDIS-1049 - Fix retrieval of empty keyspaceNotificationsConfigParameter used to keep server defaults.

Empty Strings changed to be considered null throwing an exception on getRequiredAttribute so we now set the value explicitly for this attribute when considered empty.

Original pull request: #491.

src/test/java/org/springframework/data/redis/repository/configuration/RedisRepositoryConfigurationExtensionUnitTests.java

@@ -114,6 +114,15 @@ public class RedisRepositoryConfigurationExtensionUnitTests {
 		assertThat(getKeyspaceNotificationsConfigParameter(beanDefintionRegistry)).isEqualTo((Object) "KEA");
 	}
 
+	@Test // DATAREDIS-1049
+	public void explicitlyEmptyKeyspaceNotificationsConfigParameterShouldBeCapturedCorrectly() {
+
+		metadata = new StandardAnnotationMetadata(ConfigWithEmptyConfigParameter.class, true);
+		BeanDefinitionRegistry beanDefintionRegistry = getBeanDefinitionRegistry();
+
+		assertThat(getKeyspaceNotificationsConfigParameter(beanDefintionRegistry)).isEqualTo("");
+	}
+
 	private static void assertDoesNotHaveRepo(Class<?> repositoryInterface,
 			Collection<RepositoryConfiguration<RepositoryConfigurationSource>> configs) {
 
@@ -179,6 +188,12 @@ public class RedisRepositoryConfigurationExtensionUnitTests {
 
 	}
 
+	@EnableRedisRepositories(considerNestedRepositories = true, enableKeyspaceEvents = EnableKeyspaceEvents.ON_STARTUP,
+			keyspaceNotificationsConfigParameter = "")
+	static class ConfigWithEmptyConfigParameter {
+
+	}
+
 	@RedisHash
 	static class Sample {
 		@Id String id;