Fix Javadoc warnings

src/main/java/org/apache/commons/logging/impl/Log4jApiLogFactory.java

@@ -183,6 +183,13 @@ public final class Log4jApiLogFactory extends LogFactory {
 
     private final ConcurrentMap<String, Object> attributes = new ConcurrentHashMap<>();
 
+    /**
+     * Constructs a new instance.
+     */
+    public Log4jApiLogFactory() {
+        // empty
+    }
+
     @Override
     public Object getAttribute(final String name) {
         return attributes.get(name);

src/main/java/org/apache/commons/logging/impl/ServletContextCleaner.java

@@ -50,6 +50,13 @@ public class ServletContextCleaner implements ServletContextListener {
 
     private static final Class<?>[] RELEASE_SIGNATURE = { ClassLoader.class };
 
+    /**
+     * Constructs a new instance.
+     */
+    public ServletContextCleaner() {
+        // empty
+    }
+
     /**
      * Invoked when a webapp is undeployed, this tells the LogFactory
      * class to release any logging information related to the current

src/main/java/org/apache/commons/logging/impl/Slf4jLogFactory.java

@@ -266,6 +266,13 @@ public final class Slf4jLogFactory extends LogFactory {
 
     private final ConcurrentMap<String, Object> attributes = new ConcurrentHashMap<>();
 
+    /**
+     * Constructs a new instance.
+     */
+    public Slf4jLogFactory() {
+        // empty
+    }
+
     @Override
     public Object getAttribute(final String name) {
         return attributes.get(name);