Handle pages with no /Contents from getPageContents()

The spec allows /Contents to be omitted for pages that are blank, but
QPDFObjectHandle::getPageContents() was throwing an exception in this
case.
Update TODO with some small enhancements

There have been a few enhancements requested that only affect the qpdf
command line tool and that should be relatively quick to implement.
Work out the details and acknowledge these publicly in the TODO file.
TODO