Ensure frontend functions work when they don't return a value

[pngwn]

Description

We now expect some kind of value to be passed into a 'submit' even if it is an empty array (no data). This normally happens even where there are no component inputs, the exception to this is when the frontend function returns nothing or a falsey value. Since the return value of the frontend function gets passed into the 'submit' call, this was causing an error.

I have added a default value to the problematic function to address this, and added a few unit tests to prevent regressions.

Closes: #8495

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Ensure frontend functions work when they don't return a value

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[aliabid94]

The root problem is that Payload.data is supposed to be of unknown[], but if the js function does not return anything, Payload.data gets set to undefined, which is not of type unknown[]. This will cause issues downstream, which currently is manifesting in map_data_to_params. I believe the appropriate way to fix the overall problem would be to modify process_frontend_fn to return [] when there are no output components.

[pngwn]

Yeah I can change that. I'm not sure what other downstream issues it would cause though. This has been the behaviour forever, it just hasn't been erroring until now.

[aliabid94]

Actually its probably enough to just type Payload.data as unknown[] | undefined and keep the current logic. I just meant the reason that things broke is because the typing was wrong - Payload.data could be undefined as well - and this caused an issue downstream in map_data_to_params, but if we simply fix the typing, then the typechecker should also catch anywhere else we assumed it to be a list.

[pngwn]

I think it's cleaner to always return an array from the frontend function, so I've added some logic for that. Should be good now.

[aliabid94]

awesome LGTM!