Core: SingleValueParser cannot round-trip optional null map values

[laserninja]

Apache Iceberg version

main (development), commit c4784768b

Query engine

Other: Java API

Please describe the bug

The public single-value serializer writes an optional null map value, but its parser cannot read that output.

Types.MapType type = Types.MapType.ofOptional(
    1, 2, Types.StringType.get(), Types.IntegerType.get());
Map<String, Integer> value = new HashMap<>();
value.put("a", null);
String json = SingleValueParser.toJson(type, value);
SingleValueParser.fromJson(type, json);

The JSON is {"keys":["a"],"values":[null]}. Parsing throws NullPointerException: null value in entry: a=null.

Expected: recover the same map. Iceberg permits optional map values. mapFromJson currently uses ImmutableMap.Builder, which rejects all null values.

Consumer reproduction: a StaticDataTask containing an optional null map value can be serialized with ScanTaskParser.toJson, but restoring it with ScanTaskParser.fromJson fails with the same null-value exception. The regression in #18118 verifies serialization, restoration, and consumption of the resulting rows. It fails before the fix and passes afterward.

Proposed fix: allow optional null values while retaining immutable results and rejecting null/duplicate keys and null required values. This report concerns the single-value parser, not support for non-null map column defaults, which are separately disallowed by the field builder.

Willingness to contribute

• I can contribute a fix for this bug independently

This report and its reproduction were prepared with AI assistance and verified locally.