Make Log4J logging system consistent with the others

when the logname is empty it replaces it with the root logger name.

Fixes gh-4808

spring-boot/src/main/java/org/springframework/boot/logging/java/JavaLoggingSystem.java

@@ -111,7 +111,8 @@ public class JavaLoggingSystem extends AbstractLoggingSystem {
 	@Override
 	public void setLogLevel(String loggerName, LogLevel level) {
 		Assert.notNull(level, "Level must not be null");
-		Logger logger = Logger.getLogger(loggerName == null ? "" : loggerName);
+		Logger logger = Logger
+				.getLogger(StringUtils.hasText(loggerName) ? loggerName : "");
 		logger.setLevel(LEVELS.get(level));
 	}
 

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -46,6 +46,7 @@ import org.springframework.boot.logging.Slf4JLoggingSystem;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ResourceUtils;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link LoggingSystem} for <a href="http://logging.apache.org/log4j/2.x/">Log4j 2</a>.
@@ -208,6 +209,8 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 	}
 
 	private LoggerConfig getLoggerConfig(String loggerName) {
+		loggerName = StringUtils.hasText(loggerName) ? loggerName
+				: LogManager.ROOT_LOGGER_NAME;
 		return getLoggerContext().getConfiguration().getLoggers().get(loggerName);
 	}