FELIX-6307 : Use ROOT_LOGGER_NAME instead of null

Specification of log service does not allow for null logger names.
Previously the ROOT_LOGGER_NAME was used by SCR in this case.  This
restores that behavior.
diff --git a/scr/src/main/java/org/apache/felix/scr/impl/logger/LogServiceEnabledLogger.java b/scr/src/main/java/org/apache/felix/scr/impl/logger/LogServiceEnabledLogger.java
index 6bc3c86..933b3e5 100644
--- a/scr/src/main/java/org/apache/felix/scr/impl/logger/LogServiceEnabledLogger.java
+++ b/scr/src/main/java/org/apache/felix/scr/impl/logger/LogServiceEnabledLogger.java
@@ -58,7 +58,7 @@
             else
             {
                 this.currentLogger = new OSGiLogger(
-                    factory.getLogger(bundle, null, Logger.class));
+                    factory.getLogger(bundle, Logger.ROOT_LOGGER_NAME, Logger.class));
             }
             this.trackingCount = this.loggingFactoryTracker.getTrackingCount();
         }