Return empty arrays instead of null in json-to-xml() for XPath consistency

[Copilot]

The json-to-xml() function was returning null for empty/invalid inputs, forcing downstream XPath operators to special-case null values. XPath semantics represent empty sequences as empty arrays.

Changes

• Modified three return paths in json-to-xml() to return [] instead of null:
  • Empty/falsy input strings
  • Failed JSON-to-XML conversion
  • Failed node conversion to XNode

This aligns with existing XPath functions (e.g., key()) and eliminates the need for callers to distinguish between null and empty sequences.

// Before
if (!jsonStr) {
    return null;
}

// After  
if (!jsonStr) {
    return [];
}

---

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	76.4%	2438/3191
🟡	Branches	65.21%	1316/2018
🟡	Functions	74.09%	326/440
🟡	Lines	76.78%	2305/3002

Test suite run success

576 tests passing in 44 suites.

Report generated by 🧪jest coverage report action from d988ac2