fix(avro): order "null" first in union when default is null

[mwikberg-virta]

The avro-schema generator emitted an invalid union when a property combined a non-null type with an explicit default: null (commonly produced by nullable: true + allOf composition), e.g. ["model.Foo", "null"] with "default": null. Per the Avro specification, a union's default value must match the FIRST branch of the union, so the default null is only valid when "null" is the first branch. The invalid ordering is accepted by most schema parsers but fails at read time when the default is applied during schema evolution, crashing consumers.

The Swagger Parser represents an explicit default: null as a Jackson NullNode (a non-null Java object) rather than a Java null, so toDefaultValue returned the string "null" and the field was rendered through the concrete-default branch ([<type>, "null"]). Treat an explicit null default as "no default" so the field falls through to the existing nullable-union form (["null", <type>] with "default": null), which is valid. Real (non-null) defaults are unaffected.

Extends the issue6268 test spec with a nullable scalar and an allOf-composed model reference, both using default: null, and regenerates the sample.

Closes: #24134

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Summary by cubic

Fixes Avro union ordering when a field has default: null by placing "null" first, preventing invalid schemas and runtime crashes during schema evolution.

• Bug Fixes
  • In org.openapitools.codegen.languages.AvroSchemaCodegen, treat explicit default: null (Jackson NullNode) as no concrete default in toDefaultValue, so we emit ["null", <type>] with "default": null per Avro spec.
  • Extended issue6268.yaml with a nullable scalar and an allOf-composed model using default: null; regenerated samples (AvroDefaultNullReference.avsc, updated SampleModelToTestAvroDefaultValues.avsc).
  • Real (non-null) defaults are unchanged.

^{Written for commit a210b47. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 5 files

_{Re-trigger cubic}