QtIFW

Fix install fail if there is no metadata

Installer was never able to continue from metadata download phase in
case there was no metadata in any of the packages. If metadata not
found, emit finished signal to continue installation.

Task-number:  QTIFW-1150
Change-Id: I0d3f8fb147899fff4b1a1cc0c6de7a587aae9053
Reviewed-by: Antti Kokko <antti.kokko@qt.io>