libbinder: Add test for null non-nullable binder return

Test: atest binderRpcTest
Change-Id: If10aac2b040f5de53516528821c6623c4b528cb8
diff --git a/libs/binder/tests/IBinderRpcTest.aidl b/libs/binder/tests/IBinderRpcTest.aidl
index fdd02a4..2deea82 100644
--- a/libs/binder/tests/IBinderRpcTest.aidl
+++ b/libs/binder/tests/IBinderRpcTest.aidl
@@ -24,6 +24,9 @@
     // number of known RPC binders to process, RpcState::countBinders by session
     int[] countBinders();
 
+    // Return a null binder with a non-nullable return type.
+    IBinder getNullBinder();
+
     // Caller sends server, callee pings caller's server and returns error code.
     int pingMe(IBinder binder);
     @nullable IBinder repeatBinder(@nullable IBinder binder);