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

[wing328]

based on #24395

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---

Summary by cubic

Handle null values in additionalProperties for the Java okhttp-gson generator. We now emit JsonNull during serialization and add the needed import to prevent runtime errors when additionalProperties contain nulls.

• Bug Fixes
  • Update pojo.mustache: import com.google.gson.JsonNull and add JsonNull.INSTANCE when gson.toJsonTree(value) is isJsonNull().
  • Regenerate Java okhttp-gson samples to include the new import and null-handling logic.

^{Written for commit f4a6a57. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 248 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AppleReq.java">

<violation number="1" location="samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AppleReq.java:32">
P3: The newly added `import com.google.gson.JsonNull;` in AppleReq.java is dead code — `JsonNull` is never referenced anywhere else in this file (it has no additionalProperties handling). Please gate the import emission in the Java pojo.mustache template on the same condition that produces the `JsonNull.INSTANCE` branch so regenerated samples don't carry an unused import.</violation>
</file>

<sub>Note: This PR contains a large number of files. cubic only reviews up to 200 files per PR, so some files may not have been reviewed. cubic prioritizes the most important files to review.

Re-trigger cubic</sub>

[cubic-dev-ai]

P3: The newly added import com.google.gson.JsonNull; in AppleReq.java is dead code — JsonNull is never referenced anywhere else in this file (it has no additionalProperties handling). Please gate the import emission in the Java pojo.mustache template on the same condition that produces the JsonNull.INSTANCE branch so regenerated samples don't carry an unused import.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/model/AppleReq.java, line 32:

<comment>The newly added `import com.google.gson.JsonNull;` in AppleReq.java is dead code — `JsonNull` is never referenced anywhere else in this file (it has no additionalProperties handling). Please gate the import emission in the Java pojo.mustache template on the same condition that produces the `JsonNull.INSTANCE` branch so regenerated samples don't carry an unused import.</comment>

<file context>
@@ -29,6 +29,7 @@
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
+import com.google.gson.JsonNull;
 import com.google.gson.JsonParseException;
 import com.google.gson.TypeAdapterFactory;
</file context>