In d1683032 (CPackIFW: Improve warning and docs for show page list
option, 2021-02-14), code was added to report the current QtIFW version.
That code didn't check that there was a Generator object before using it,
resulting in a scanbuild warning about a potential null pointer dereference.
Add a check and output a more general warning message if we don't have
the current QtIFW version available to report.