Register connection types explicitly if not already registered

Follows up on qtbase:b0f495735c30e400c9749c8b9f9823dbb56dfbd9, which
removed the implicit metatype registration of QObject pointers and
gadgets.

If we have arguments but get no type IDs, fall back to an explicit
registration of the types via QMetaType::fromType(), which we can use
here.

Pick-to: 6.12
Change-Id: I7a534f219988bc008492065892f1ae8bf38fd7e6
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>