[Dentosal]

Follow-up to #1008.

Previously, minting zero units of a nonexisting asset would write that to the storage. This PR ignores zero-amount mints, although it still produces a receipt for them. The change should be mostly inconsequential, but makes the behavior consistent with burn opcode.

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

Low Risk
Small, well-scoped change to mint balance mutation semantics for the amount == 0 edge case, with test coverage updates and no new data paths.

Overview
Updates the mint opcode handling to skip reading/updating contract asset balances and charging new-storage gas when amount == 0, preventing creation of zero-balance entries for previously-nonexistent assets while still emitting a Receipt::Mint.

Adjusts mint-related tests to treat missing balances as 0 (via unwrap_or(0)) to reflect the new no-write behavior, and adds a changelog entry documenting the semantic change.

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