fix: Reject empty Decimal bytes during schema-aware serialization (#684)

* fix: Reject empty Decimal bytes during schema-aware serialization

Reject empty bytes-backed Decimal values before direct or selected union serialization writes bytes. Preserve ordinary bytes, fixed branches, and union selection.

Generated-by: OpenAI Codex

* fix: use contextual errors for empty Decimal bytes

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Co-authored-by: Priyansh Rana <Priyansh@Parimals-Mac.local>
Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>