Summary
-------

Add explicit constructors to two classes in java.xml using default constructors.

Problem
-------

Missing default constructors in org.xml.sax.HandlerBase and org.xml.sax.helpers/DefaultHandler.

Solution
--------

Add explicit default constructors to the two classes.

Specification
-------------

    --- a/src/java.xml/share/classes/org/xml/sax/HandlerBase.java
    +++ b/src/java.xml/share/classes/org/xml/sax/HandlerBase.java
    @@ -58,7 +58,10 @@
     public class HandlerBase
         implements EntityResolver, DTDHandler, DocumentHandler, ErrorHandler
     {
    -
    +    /**
    +     * Constructs a {@code HandlerBase}.
    +     */
    +    public HandlerBase() {}

         ////////////////////////////////////////////////////////////////////
         // Default implementation of the EntityResolver interface.
    
    --- a/src/java.xml/share/classes/org/xml/sax/helpers/DefaultHandler.java
    +++ b/src/java.xml/share/classes/org/xml/sax/helpers/DefaultHandler.java
    @@ -1,5 +1,5 @@
     /*
    - * Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
    + * Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
      * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
      *
      * This code is free software; you can redistribute it and/or modify it
    @@ -70,7 +70,10 @@
     public class DefaultHandler
         implements EntityResolver, DTDHandler, ContentHandler, ErrorHandler
     {
    -
    +    /**
    +     * Constructs a {@code DefaultHandler}.
    +     */
    +    public DefaultHandler() {}

         ////////////////////////////////////////////////////////////////////
         // Default implementation of the EntityResolver interface.