PDFBOX-2417, PDFBOX-2418: find namespaces that are not at top level to avoid "Schema is not set in this document" error, as suggested by Ralf Hauser; use isEmpty()

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1635064 13f79535-47bb-0310-9956-ffa450edef68