[OSX] [Demobrowser] Print to PDF does nothing

When trying to save a web page as PDF, using the demobrowser, the native Mac print dialog appears. After choosing save to PDF instead of printing, and entering a file name, the dialog is closed and nothing happens.

Traced it down to the following change https://codereview.qt-project.org/#/c/110563/2

Because the output file name is not set, the demobrowser checks that the name is empty, and does not invoke save to pdf behavior.

Just for testing, if the commit is reverted, the pdf file is successfully saved.
But obviously that will probably break stuff for the underlying issue of the change.