swap default variable values to null/undef

src/main/java/carpet/script/Expression.java

@@ -738,7 +738,7 @@ public class Expression
         }
         var = c.host.getGlobalVariable(module, name);
         if (var != null) return var;
-        var = (_c, _t ) -> Value.ZERO.reboundedTo(name);
+        var = (_c, _t ) -> _c.host.strict ? Value.UNDEF.reboundedTo(name) : Value.NULL.reboundedTo(name);
         setAnyVariable(c, name, var);
         return var;
     }