[Variant] Unshredded VariantArray producers annotate the value field as nullable (spec requires required)

[sdf-jkl]

Which issue does this PR close?

• Closes [Variant] Unshredded VariantArray producers annotate the value field as nullable (spec requires required) #10315.

• let's wait till [Variant] make value mandatory field for VariantArray/ShreddingState #10318 ships

Rationale for this change

• Parquet spec required unshredded VariantArray to be non-nullable. Other readers might reject our impl because unshred_variant and variant_get as_type None return nullable unshredded Variant.

What changes are included in this PR?

• builds on [Variant] make value mandatory field for VariantArray/ShreddingState #10318
• Add changes to unshred_variant and variant_get to return non-nullable unshredded Variant

Are these changes tested?

• yes, added new unit tests and fixed some old ones

Are there any user-facing changes?

• Yes, the nullability now conforms the spec

[sdf-jkl]

@klion26 Please take a look 🙏