Fix issue with keepLatest and empty values

[wagenet]

tl;dr:

• use the value on the first run of the keepLatest, if available (instead of previous, which is always undefined)
• add tests clarifying behavior

---

The way to provide a default value when the value of keepLatest is false is to use another getter.

This PR adds a couple tests clarifying that behavior:

class Demo {
  @use latest = keepLatest({
    value: () => this.data.value?.meta,
    when: () => this.data.isLoading,    
  })

  get theData() {
    return this.latest ?? 'default value';
  }
}

It's important to not do

   value: () => this.data.value?.meta ?? {},

because that means falsey responses (on data), will resolve to {}, and cause content flashes.

So to keep the latest value, you want to let the data response be what it actually is.

In traditional logic, this could maybe be expressed this way:

value ?? default ?? previous

when default is always truthy
vs

(value ?? previous) ?? default

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[changeset-bot]

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Latest commit: 7970e89

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