Restore suppressValue to memberLookup

Consider the following code:

```django
{% for i in foo.bar %} {% endfor %}
```
if `foo` is defined as `{}` in the current context. The produced code would be something like:

```js
var t_1 = runtime.memberLookup((l_foo),"bar", env.autoesc);
for(var t_2=0; t_2 < t_1.length; t_1++) {
// ...
}
```

`t_1` would otherwise contain `undefined`, meaning that `t_1.length` would throw an exception.