[jnigen] Support use arenas on nullable JObjects

pkgs/jni/lib/src/jobject.dart

@@ -219,15 +219,15 @@ class JObject {
   void releasedBy(Arena arena) => arena.onReleaseAll(release);
 }
 
-extension JObjectUseExtension<T extends JObject> on T {
+extension JObjectUseExtension<T extends JObject?> on T {
   /// Applies [callback] on this object and then delete the underlying JNI
   /// reference, returning the result of [callback].
   R use<R>(R Function(T) callback) {
     try {
       final result = callback(this);
       return result;
     } finally {
-      release();
+      this?.release();
     }
   }
 }

pkgs/jni/test/jobject_test.dart

@@ -175,6 +175,8 @@ void run({required TestRunnerCallback testRunner}) {
       });
     });
     expect(randomInt, lessThan(15));
+    const JObject? nullableJObject = null;
+    expect(nullableJObject.use((_) => 'foo'), equals('foo'));
   });
 
   // The JObject and JClass have NativeFinalizer. However, it's possible to