Fix usage of "compact" Variant discriminators serialization

Why it matters

This fix resolves inconsistencies in how Variant discriminators are serialized using the "compact" format in MergeTree tables, improving storage efficiency and consistency when this optimization can be applied.

How to use it

The feature is applied internally during Variant type serialization in MergeTree engines and does not require explicit user action. Users benefit automatically from improved compact serialization of Variant discriminators after upgrading to this version.