[LOG4J2-3564] Fix NPE when root logger level is null

`LogManager.getGlobal().getLevel()` is not a good fallback for the case
when all JUL logger levels are null, since its level is usually null.

Author: Piotr P. Karwasz

log4j-to-jul/src/main/java/org/apache/logging/log4j/tojul/JULLogger.java

@@ -32,7 +32,7 @@
  *
  * @author <a href="http://www.vorburger.ch">Michael Vorburger.ch</a> for Google
  */
-public final class JULLogger extends AbstractLogger {
+final class JULLogger extends AbstractLogger {
     private static final long serialVersionUID = 1L;
 
     private final Logger logger;
@@ -162,7 +162,8 @@ private java.util.logging.Level getEffectiveJULLevel() {
         }
         // This is a safety fallback that is typically never reached, because usually the root Logger.getLogger("") has
         // a Level.
-        return Logger.getGlobal().getLevel();
+        // Since JDK 8 the LogManager$RootLogger does not have a default level, just a default effective level of INFO.
+        return java.util.logging.Level.INFO;
     }
 
     private boolean isEnabledFor(final Level level, final Marker marker) {

log4j-to-jul/src/main/java/org/apache/logging/log4j/tojul/package-info.java

@@ -21,7 +21,7 @@
  * @author <a href="http://www.vorburger.ch">Michael Vorburger.ch</a> for Google
  */
 @Export
-@Version("2.20.1")
+@Version("3.0.0")
 package org.apache.logging.log4j.tojul;
 
 import org.osgi.annotation.bundle.Export;

log4j-to-jul/src/test/java/org/apache/logging/log4j/tojul/JULLoggerTest.java

@@ -0,0 +1,34 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.logging.log4j.tojul;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.apache.logging.log4j.Level;
+import org.junit.jupiter.api.Test;
+
+public class JULLoggerTest {
+
+    @Test
+    public void testNotNullEffectiveLevel() {
+        // Emulates the root logger found in Tomcat, with a null level
+        // See: https://bz.apache.org/bugzilla/show_bug.cgi?id=66184
+        final java.util.logging.Logger julLogger = new java.util.logging.Logger("", null) {};
+        final JULLogger logger = new JULLogger("", julLogger);
+        assertEquals(Level.INFO, logger.getLevel());
+    }
+}