Core: Parse optional null map values in SingleValueParser

[laserninja]

Fixes #18117.

SingleValueParser.toJson can serialize an optional null map value, but fromJson fails on that output because ImmutableMap.Builder rejects nulls.

Build a LinkedHashMap and return an unmodifiable view to preserve iteration order and immutability while accepting optional null values. Explicitly reject null keys, duplicate keys (including an earlier null value), and null required values with argument errors.

Validation on JDK 17: the five new regressions failed before the fix. TestSingleValueParser, TestSchemaParser, TestDataTaskParser, TestContentFileParser, and TestExpressionParser passed afterward, along with core formatting, checkstyle, and revapi checks.

Consumer regression: a StaticDataTask containing an optional null map value is serialized through ScanTaskParser, restored, and its rows consumed. This test fails with the original parser (null-value exception) and passes with the fix. The full TestDataTaskParser suite and core formatting/checkstyle checks pass on JDK 17.

Additional local validation at 76b236c67: the full core check passed on JDK 17, with 8,258 passing tests, 550 skipped tests, and zero failures or errors. Apache RAT (dev/check-license) and the full Java CI command (./gradlew check -DsparkVersions= -DflinkVersions= -DkafkaVersions= -Pquick=true -x javadoc -x spotlessCheck) also passed. The latter includes the OpenAPI tests and specification validation; it does not run the Spark/Flink/Kafka test matrices.

---

AI Disclosure

• Model: GPT-6
• Platform/Tool: Codex
• Human Oversight: reviewed
• Prompt Summary: Reproduce and fix optional null map value parsing, preserve validation and immutability, and create an issue and PR.

[uros-b]

Nice, thank you @laserninja! This closes a real toJson/fromJson asymmetry (ImmutableMap.Builder rejects the null values toJson already emits); the LinkedHashMap + unmodifiableMap swap is well-chosen, validation correctly ordered (null-key → null-required → duplicate), round-trip verified both ways with a ScanTaskParser consumer regression.

[uros-b]

For further review, cc @nastra who has more context in core serialization