fix(runtime-core): watching multiple values - handle undefined as initial values

[webfansplz]

Closes #7070

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[LinusBorg]

@KaelWD Looks good, wdyt?

[KaelWD]

Looks good, I wonder if there's a way to do const INITIAL_WATCHER_VALUE_ARRAY = [] though instead of all the extra conditions and new arrays. Before #5033 it was expect(oldVal).toMatchObject([]) which would also be fine unless someone cares about .length of that array.

[yyx990803]

Thanks for the PR! I believe the length of the initial old value array does not matter, so an empty array should suffice. See 23e85e2