DefaultGroovyMethods File[] null check

There is an open bug in the JDK: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4803836. File.listFiles() returns null when for the System Volume Information directory contrary to what the JDK reports. Several methods in the DefaultGroovyMethods class correctly expect the File[] not to be null according to the JDK. Rod Cope has added a null check to handle the bug in the JDK to the following methods: eachFile, eachFileRecurse, eachDir, and eachFileMatch. No testcase is provided since this must be run against an active drive and the code changes are simply null checks ( if (files != null) ).