[gbinder] use BINDER_TYPE_BINDER for NULL local object

3 reasons:
- This is what encode_remote_object() does. I see no reason a NULL local
  object should be encoded differently than a NULL remote object.
- This is what Parcel.cpp does when flattening a NULL binder [1]. This
  is contrary to what is said in PR #99 [2]; I'm not sure why PR #99
  said it uses BINDER_TYPE_HANDLE.
- More importantly, BINDER_TYPE_HANDLE number 0 does NOT represent a
  NULL binder. According to the comment at [3], handle number 0 actually
  represent the context manager. So, by sending BINDER_TYPE_HANDLE
  number 0, we're sending context manager, not a NULL binder.

[1]: https://android.googlesource.com/platform/frameworks/native/+/refs/tags/android-14.0.0_r1/libs/binder/Parcel.cpp#277
[2]: https://github.com/mer-hybris/libgbinder/pull/99
[3]: https://android.googlesource.com/platform/frameworks/native/+/refs/tags/android-14.0.0_r1/libs/binder/ProcessState.cpp#336

src/gbinder_io.c

@@ -175,12 +175,10 @@ GBINDER_IO_FN(encode_local_object)(
     struct flat_binder_object* dest = out;
 
     memset(dest, 0, sizeof(*dest));
+    dest->hdr.type = BINDER_TYPE_BINDER;
     if (obj) {
-        dest->hdr.type = BINDER_TYPE_BINDER;
         dest->flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
         dest->binder = (uintptr_t)obj;
-    } else {
-        dest->hdr.type = BINDER_TYPE_HANDLE;
     }
     if (protocol->finish_flatten_binder) {
         protocol->finish_flatten_binder(dest + 1, obj);