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/main/java/org/springframework/data/redis/repository/configuration/EnableRedisRepositories.java

@@ -168,7 +168,7 @@ public @interface EnableRedisRepositories {
 
 	/**
 	 * Configure the {@literal notify-keyspace-events} property if not already set. <br />
-	 * Use an empty {@link String} to keep <b>not</b> alter existing server configuration.
+	 * Use an empty {@link String} to keep (<b>not</b> alter) existing server configuration.
 	 *
 	 * @return {@literal Ex} by default.
 	 * @since 1.8

src/main/java/org/springframework/data/redis/repository/configuration/RedisRepositoryConfigurationExtension.java

@@ -144,7 +144,7 @@ public class RedisRepositoryConfigurationExtension extends KeyValueRepositoryCon
 				.addPropertyValue("enableKeyspaceEvents",
 						configuration.getRequiredAttribute("enableKeyspaceEvents", EnableKeyspaceEvents.class)) //
 				.addPropertyValue("keyspaceNotificationsConfigParameter",
-						configuration.getRequiredAttribute("keyspaceNotificationsConfigParameter", String.class)) //
+						configuration.getAttribute("keyspaceNotificationsConfigParameter", String.class).orElse("")) //
 				.getBeanDefinition();
 	}