[Variant] Implement `ShreddingState::AllNull` variant

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

• part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736

We introduced ShreddingState in

• [Variant] Add variant_get and Shredded VariantArray #8021

And now we need to expand it to handle the null, null case

Describe the solution you'd like

Basically, add ShreddingState::AllNull and then implement the relevant APIs in variant_get and OutputBuilder, with tests

For a future FullyShreddedAllNull variant (neither value nor typed_value present), would we still need to store the metadata even tho it's never actually used? 🤔

Originally posted by @scovich in #8021 (comment)

[codephage2020]

take

[alamb]

label_issue.py automatically added labels {'parquet'} from #8021