This is always an error, and we should not silently return an empty
string and pretend that everything is in order.
Drive-by change: Add a comment stating what createResponseFile()
returns.
Pick-to: 6.3
Change-Id: I4ee940cfac826c7ae5d15e977b692f1368ab29ea
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>