[Hollujay]

Summary

• Fix the setOptions decoder handling of absent optional numeric fields.
• Prevent literal null from appearing in signer-facing transaction summaries.
• Preserve explicit values, including 0, as real transaction instructions.
• Update the shared decoder fixture and regression coverage.

Problem

@stellar/stellar-sdk represents absent Optional<uint32> values in
SetOptionsOp as null.

The decoder previously checked those fields with !== undefined, causing
absent fields to render as literal null.

That inaccurately suggested the transaction was modifying account properties
that were actually untouched.

Fix

The six affected guards now use != null:

• masterWeight
• lowThreshold
• medThreshold
• highThreshold
• setFlags
• clearFlags

Absent fields are omitted from the summary, while explicit values such as
masterWeight: 0 remain visible.

Testing

• npm test — 56/56 passed
• npm run typecheck — passed
• npm run lint — passed
• git diff --check — clean

Scope

This PR only changes the setOptions decoder behavior and its regression
coverage.

No signing, submission, database, dependency, or CI behavior was changed.

The corresponding coordinator-web fix will be landed separately to preserve
the two-repository decoder parity.