Handle Variant column NULL values (0xFF discriminator)

[thearyanahmed]

This PR tries to fix #392 by adding support for NULL values in Variant columns by handling the 0xFF discriminator.

Variant is implicitly nullable — NULL is encoded as discriminator 0xFF, unlike Nullable(T) which uses a separate leading byte. The client now distinguishes between these two null encodings via a generic NullEncoding enum on the SchemaValidator trait, allowing Option<MyEnum> to work with Variant columns.

Checklist

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later

[CLAassistant]

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