add test for calling CallStack exceptionStack without previous error

tests/unit/src/unitstd/haxe/CallStack.unit.hx

@@ -10,3 +10,11 @@ try {
     var stack = haxe.CallStack.exceptionStack();
     Std.is(stack, Array) == true;
 }
+#if js
+var old = @:privateAccess haxe.CallStack.lastException;
+@:privateAccess haxe.CallStack.lastException = null;
+var stack = haxe.CallStack.exceptionStack();
+Std.is(stack, Array) == true;
+stack.length == 0;
+@:privateAccess haxe.CallStack.lastException = old;
+#end