fix(queryObserver): Load currentResult for placeholder data after first update

[DennyScott]

Only the this.currentQuery data is checked for stored data when updating results, where placeholder data is stored only in this.currentResult on update. So if we hit the placeholder conditional, we check to see if there is data already in the currentResult before running the placeholder fn.

It could be argued that this logic could be pulled out of the placeholder conditional, since if any other flow ever does this, it would also need this data, but unsure of the effect of this change.

Fixes #1522

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[boschni]

Will be fixed by #1775 . It cannot be guaranteed the function will only trigger once though, but you can provide a memoized function (or the result of a memoized function) as placeholder data to reduce resource usage if needed.