[24.1] Fix collectionElementsStore's hasLoadingCollectionElementsError computed.

[dannon]

As it was, false was being used as an index instead of the fallback return value -- just a minor typo from #18756.

No idea if this manifested as anything wonky in the app -- just something I noticed working on the vue3 branch.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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[bgruening]

Thanks @dannon