[JVM] Restore old implementation of 'lazy gather'

The performance optimization of 971174f broke things on the JVM
backend. This is a known problem with nqp::null getting HLLized
showing up again -- cmp. #1613

src/core.c/Rakudo/Iterator.pm6

@@ -2234,17 +2234,29 @@ class Rakudo::Iterator {
         }
 
         method pull-one() is raw {
+#?if !jvm
             nqp::ifnull(
               $!iterator,
               $!iterator := $!iterable.iterator
             ).pull-one
+#?endif
+#?if jvm
+            $!iterator := $!iterable.iterator unless $!iterator.DEFINITE;
+            $!iterator.pull-one
+#?endif
         }
 
         method push-exactly(\target, int $n) {
+#?if !jvm
             nqp::ifnull(
               $!iterator,
               $!iterator := $!iterable.iterator
             ).push-exactly(target, $n);
+#?endif
+#?if jvm
+            $!iterator := $!iterable.iterator unless $!iterator.DEFINITE;
+            $!iterator.push-exactly(target, $n);
+#?endif
         }
 
         method is-lazy(--> True) { }