fix(core): nullable array types in 3.1.x specs

[joscha]

Currently, in 3.1 specs, the follwing:

      properties:
        arrayDataOrNull:
          items:
            $ref: "#/components/schemas/SomeObject"
          type:
            - array
            - 'null'

yields:

'arrayDataOrNull': Array<SomeObject>

where it should yield:

'arrayDataOrNull': Array<SomeObject> | null;

It seems either nullability is not determined correctly in core and/or the typescript generator doesn't interpret it correctly.

Real-life example: planet-a-ventures/affinity-node#49
Discussion: https://openapi-generator.slack.com/archives/CLSB0U0R5/p1727342905017529

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[joscha]

This is the broken part

[joscha]

Should be:

Suggested change

	'arrayDataOrNull': Array<SomeObject>;
	'arrayDataOrNull': Array<SomeObject> | null;

[joscha]

This line is the fix - it now has the | null

[joscha]

This makes sure that all the generated code collapses in the github diff.

[joscha]

Further up, we fix up the nullable flag when the types contain the new 3.1 null type:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java

Lines 1249 to 1252 in 29c3b02

	if (schema.getTypes().contains("null")) {
	schema.setNullable(true);
	schema.getTypes().remove("null");
	}

but then, if there's a single type left AND it is an array schema, we forget to copy this property to the cloned ArraySchema. This line makes sure that the nullable flag is transferred onto the schema clone.

[joscha]

This issue is likely to happen again in the future when/if a new property is added that is in 3.1+ but not 3.0.x - we could possibly introduce a clone or transfer function somewhere which would make this a bit more obvious, but it's not a guarantee either.

[macjohnny]

thanks!