A default-constructed QPluginLoader erroneously reports that the
load hints are empty. However, setting a filename would then
automatically set the PreventUnload hint, surprising the user.
Return the correct flags instead.
Amends 494376f980
Change-Id: I7a95964cb680afd3adf2f71ed73d2f93023238f2
Fixes: QTBUG-100416
Pick-to: 5.15 6.2 6.3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>