[Variant] extend shredded null handling for arrays (#9599)

# Which issue does this PR close?

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- Closes #8400.

# Rationale for this change

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# What changes are included in this PR?

- Added `AppendNullMode` enum supporting all semantics.
- Replaced the bool logic to the new enum
- Fix test outputs for List Array cases

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# Are these changes tested?
- Added unit tests
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# Are there any user-facing changes?

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Author: sdf-jkl