LibJS: Allow cells to mark null pointers

This simplifies the cell visiting functions by letting them not worry
about the pointers they pass to the visitor being null.

Libraries/LibJS/Runtime/Cell.h

@@ -49,8 +49,11 @@ public:
 
     class Visitor {
     public:
-        virtual void visit(Cell*) = 0;
+        void visit(Cell*);
         void visit(Value);
+
+    protected:
+        virtual void visit_impl(Cell*) = 0;
     };
 
     virtual void visit_children(Visitor&) {}