[jni] Throw NullPointerException for null JNI parameters

[sagar-h007]

This PR adds generated null checks to JNI wrapper functions and throws a proper NullPointerException when required JNI parameters are null. This prevents native crashes and ensures errors are surfaced to Dart as JniException instead of failing silently.

What I did

•Updated the JNI C wrapper generator to add null checks for JNI reference and ID parameters
•Changed null handling to actively throw java/lang/NullPointerException
•Skipped null checks for JNI functions that are defined to accept null
•Regenerated JNI wrapper C files
•Added tests to verify null parameters now result in Dart-side exceptions

What this does

Before, passing null into some generated JNI calls could crash native code or return incorrect results. With this change, null inputs are detected early and reported as exceptions, making failures safer and easier to debug.

Fixes: #2943

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[liamappelbe]

It looks like you're changing the code-gen script that generates the JNI bindings, which is fine. But users don't run that code-gen script, they just use the generated bindings, so make sure to run the code-gen and check in the result.

Also, make sure to dart format and dart analyze your code before you upload it.

[sagar-h007]

@liamappelbe I’ll run the code generation script and check in the regenerated JNI bindings. I’ll also make sure everything is formatted and dart analyze is clean before updating the PR.

[liamappelbe]

I’ll run the code generation script and check in the regenerated JNI bindings

I don't see any updates to the generated bindings.