[reactive-element] queryAssignedElements is initially empty

### Which package(s) are affected?

Lit Core (lit / lit-html / lit-element / reactive-element)

### Description

`queryAssignedElements` is initially empty when a slot is used.

```
@queryAssignedElements({ slot: 'foo' })
fooEl!: Array;
```

Add an intital value for that slot:

```

```

`this.fooEl.length` is `0`.

### Reproduction

[Example in Playground](https://lit.dev/playground/#project=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-SGVsbG8sICR7dGhpcy5uYW1lfSE8L3A-PHNsb3QgbmFtZT1cImZvb1wiPjwvc2xvdD4ke3RoaXMuZm9vRWwubGVuZ3RofWA7XG4gIH1cbn1cbiJ9LHsibmFtZSI6ImluZGV4Lmh0bWwiLCJjb250ZW50IjoiPCFET0NUWVBFIGh0bWw-XG48aGVhZD5cbiAgPHNjcmlwdCB0eXBlPVwibW9kdWxlXCIgc3JjPVwiLi9zaW1wbGUtZ3JlZXRpbmcuanNcIj48L3NjcmlwdD5cbjwvaGVhZD5cbjxib2R5PlxuICA8c2ltcGxlLWdyZWV0aW5nIG5hbWU9XCJXb3JsZFwiPjxkaXYgc2xvdD1cImZvb1wiPmZvbzwvZGl2Pjwvc2ltcGxlLWdyZWV0aW5nPlxuPC9ib2R5PlxuIn0seyJuYW1lIjoicGFja2FnZS5qc29uIiwiY29udGVudCI6IntcbiAgXCJkZXBlbmRlbmNpZXNcIjoge1xuICAgIFwibGl0XCI6IFwiXjIuMC4wXCIsXG4gICAgXCJAbGl0L3JlYWN0aXZlLWVsZW1lbnRcIjogXCJeMS4wLjBcIixcbiAgICBcImxpdC1lbGVtZW50XCI6IFwiXjMuMC4wXCIsXG4gICAgXCJsaXQtaHRtbFwiOiBcIl4yLjAuMFwiXG4gIH1cbn0iLCJoaWRkZW4iOnRydWV9XQ)

### Workaround

Triggering manually a `document.querySelector('simple-greeting').requestUpdate()` gives the expected result.

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

2.6.1

### Browser/OS/Node environment

-

Start with the reactive-element queryAssignedElements decorator and reproduce the initial slotted-element case in the linked Lit Playground. Trace why the value is empty before requestUpdate(), then verify that the initial value contains the assigned

Written by the indexing model from the issue text.