Fix starter/stopper contextual check.

It was always not checking for the right thing. Somehow we got away
with it on Parrot and JVM, but Moar tripped. So, correct it. It will
save some spurious calls to !LITERAL on all backends.

src/HLL/Grammar.nqp

@@ -367,16 +367,16 @@ position C<pos>.
 
     method starter() {
         my $start := $*QUOTE_START;
-        nqp::isnull($start)
-            ?? self.'!cursor_start_fail'()
-            !! self.'!LITERAL'($start)
+        nqp::isconcrete($start)
+            ?? self.'!LITERAL'($start)
+            !! self.'!cursor_start_fail'()
     }
 
     method stopper() {
         my $stop := $*QUOTE_STOP;
-        nqp::isnull($stop)
-            ?? self.'!cursor_start_fail'()
-            !! self.'!LITERAL'($stop)
+        nqp::isconcrete($stop)
+            ?? self.'!LITERAL'($stop)
+            !! self.'!cursor_start_fail'()
     }
 
     our method split_words(str $words) {