[java] fix CustomTypeAdapterFactory NPE-style crash on null additionalProperties value

[chetan26]

Title:

[java] fix CustomTypeAdapterFactory NPE-style crash on null additionalProperties value
Body:

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  ./mvnw clean package
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh

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Description

Follow-up to #14404 / #17812.

That fix made pojo.mustache's generated CustomTypeAdapterFactory.write() handle JSON arrays in additionalProperties, but the else branch still calls getAsJsonObject() unconditionally:

JsonElement jsonElement = gson.toJsonTree(entry.getValue());
if (jsonElement.isJsonArray()) {
obj.add(entry.getKey(), jsonElement.getAsJsonArray());
} else {
obj.add(entry.getKey(), jsonElement.getAsJsonObject());
}

If an additionalProperties: true model has an undeclared-property entry whose value is null (e.g. an unknown field the server returned as JSON null), gson.toJsonTree(null) returns JsonNull.INSTANCE, and .getAsJsonObject() on that throws:

java.lang.IllegalStateException: Not a JSON Object: null at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:90)

This adds an isJsonNull() check ahead of the array/object branches, mirroring the same pattern #17812 used for arrays:

JsonElement jsonElement = gson.toJsonTree(entry.getValue());
if (jsonElement.isJsonNull()) {
  obj.add(entry.getKey(), JsonNull.INSTANCE);
} else if (jsonElement.isJsonArray()) {
  obj.add(entry.getKey(), jsonElement.getAsJsonArray());
} else {
  obj.add(entry.getKey(), jsonElement.getAsJsonObject());
}

Fixes #24393

How to validate

AdditionalPropertiesClass model = new AdditionalPropertiesClass();
model.putAdditionalProperty("unknownField", null);
model.toJson(); // previously threw IllegalStateException: Not a JSON Object: null

Regenerated all java-okhttp-gson* sample configs (bin/configs/java-okhttp*.yaml) — diff is uniform and confined to the additionalProperties block (new JsonNull import + the isJsonNull() branch) in every model that has additionalProperties: true.
Compiled the regenerated samples/client/petstore/java/okhttp-gson project (mvn compile) — builds clean.
Ran the existing JavaClientCodegenTest suite: 263/263 passed, no regressions.
Verified end-to-end against the regenerated AdditionalPropertiesClass: toJson() no longer throws for a null-valued additional property.

---

Summary by cubic

Fix crash in generated CustomTypeAdapterFactory.write() for Java okhttp-gson models when additionalProperties contains null. Nulls are now serialized as JsonNull to avoid IllegalStateException.

• Bug Fixes
  • Add isJsonNull() branch and write JsonNull.INSTANCE before array/object handling in additionalProperties in pojo.mustache.
  • Regenerate java-okhttp-gson* samples (adds JsonNull import and null branch); remove unused imports; samples compile and existing tests pass.

^{Written for commit a5d0787. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 248 files

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[chetan26]

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[cubic-dev-ai]

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[cubic-dev-ai]

No issues found across 146 files

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