polish(ast): prefer undefined over empty arrays

[yaacovCR]

see: #2405 (comment)

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[yaacovCR]

Well, it works!

It would seem that it's more accurate to use undefined when an optionalMany(...) is skipped entirely and an empty array when it is found, but has an empty list. So, it would seem that this PR is technically more correct.

Peeking around at https://astexplorer.net/ it seems that TS parser has some empty arrays, some undefined, while for Babel I didn't on a very cursory look see any undefined.

But still not 100% sure this is too compelling, with the status quo being somewhat compelling! Comments welcome!