Use a smarter semaphore in Supply.produce

Since the produced values can never be an nqp::null, use nqp::null as a
semaphore to indicate the first value has not yet been seen.  Removes the
need for a boolean semaphore.

src/core.c/Supply-factories.pm6

@@ -494,17 +494,11 @@
 
     method produce(Supply:D: &with) {
         supply {
-            my $first := True;
-            my $reduced := Nil;
+            my $reduced := nqp::null;
             whenever self -> \value {
-                if $first {
-                    $reduced := value;
-                    $first := False;
-                }
-                else {
-                    $reduced := with($reduced, value);
-                }
-                emit $reduced;
+                emit $reduced := nqp::isnull($reduced)
+                  ?? value
+                  !! with($reduced, value);
             }
         }
     }