[Dentosal]

Previously, burning zero units of a nonexisting asset would write that to the storage. This PR ignores zero-amount burns, although it still produces a receipt for them. In addition, when all assets with some specific have been burned, clears the slot instead of writing a zero there.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

[cursor]

PR Summary

High Risk
Changes BURN state transition semantics for contract asset balances (skips storage writes for zero-amount burns and deletes entries when balance hits zero), which can affect persisted state and consensus/state roots. Risk is concentrated in opcode execution and storage mutation paths.

Overview
Minimizes contract asset storage writes during the BURN opcode by skipping balance updates when amount == 0 while still emitting a burn receipt.

When a burn brings an asset balance to exactly zero, it now removes the (contract, asset_id) entry instead of persisting a 0 value, via a new ContractsAssetsStorage::contract_asset_id_balance_remove helper. Tests were updated to treat missing balances as 0 (unwrap_or(0)).

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