DERBY-6396: NullPointerException in DirFile

Check that the array returned by File.list() is non-null before
attempting to iterate through it. Null is a valid return value.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1537394 13f79535-47bb-0310-9956-ffa450edef68