[Parquet] Variant `Null` type `typed_value` field is supported

[sdf-jkl]

Describe the bug, including details regarding any error messages, version, and platform.

Tbd - apache/arrow#50810

Per the discussion here - apache/arrow#50622 - Variant shouldn't support Null type typed_value array.

Nulls meaning in the shredding spec:
https://github.com/apache/parquet-format/blob/2076361bb64e2de9ca6a8d06eda025a6fa4e9df6/VariantShredding.md?plain=1#L68-L73

Component(s)

Parquet