Use target type as default default for coercers

This way we don't coerce optional parameters when they're not
provided, avoiding uwanted crashes and warnings.

Fixes RT#130479: https://rt.perl.org/Ticket/Display.html?id=130479

src/Perl6/Actions.nqp

@@ -8738,7 +8738,8 @@ class Perl6::Actions is HLL::Actions does STDActions {
                             $var.default(QAST::SVal.new( :value('') ));
                         }
                         else {
-                            $var.default(QAST::WVal.new( :value($nomtype) ));
+                            $var.default: QAST::WVal.new: value =>
+                              nqp::ifnull($coerce_to, $nomtype);
                         }
                     }
                 }