reactive dependency not referenced can't set to undefined initially

[tanhauhau]

Fixes #5538

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Tests

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[Conduitry]

The failures here look to be all js sample tests. I haven't looked into whether they seem to be expected changes.

[tanhauhau]

let me update the js sample tests.

let me explained a bit on what's changed.

<script>
   let a = 1;
   $: b = a;
   function update() {
       a = undefined;
   }
</script>
{ b }

because a is not referenced in the template, it wasn't included in the initial list of variables returned from instance. which means ctx[a_index] is undefined, even though value of a is not.

so when we call update, which $$invalidate(a_index, a = undefined), it didn't registered it as changed, since ctx[a_index] === undefined.

so, the changes is to include a into the initial list of variables. therefore there's some changes in the js samples.