fix the Thread Network fuzz testing error

The current fuzz testing comunicates with the Thread radio chip
simulation program `ot-rcp`. The 'ot-rcp' is not installed by all
devices, which causes the fuzz testing program is unable to find
the program '/vendor/bin/ot-rcp'.

This commit enables the HAL to talk to the '/dev/null' to avoid
'/vendor/bin/ot-rcp' not being found.

Bug: 289627270
Test: Run the fuzz test on cuttlefish
Change-Id: If410d7d75abe05c7c2d2c82fa0450e925b9a48bb
diff --git a/threadnetwork/aidl/default/fuzzer.cpp b/threadnetwork/aidl/default/fuzzer.cpp
index 512708d..fb6e548 100644
--- a/threadnetwork/aidl/default/fuzzer.cpp
+++ b/threadnetwork/aidl/default/fuzzer.cpp
@@ -22,7 +22,7 @@
 using android::fuzzService;
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-    char url[] = "spinel+hdlc+forkpty:///vendor/bin/ot-rcp?forkpty-arg=2";
+    char url[] = "spinel+hdlc+null:///dev/null";
     auto service = ndk::SharedRefBase::make<ThreadChip>(url);
 
     fuzzService(service->asBinder().get(), FuzzedDataProvider(data, size));