Fix NullPointerException on NsdService

The NsdService will throw a NPE if a new client is registered
with a null INsdManagerCallback object. To avoid this, perform
a null check before registering a new client and throw an
IllegalArgumentException if the callback is null.

Bug: 293285797
Test: atest FrameworksNetTests
Change-Id: Id61e27873591031c3fe383879aee0d40eebc08b3

tests/unit/java/com/android/server/NsdServiceTest.java

@@ -1585,6 +1585,20 @@ public class NsdServiceTest {
         lockOrder.verify(mMulticastLock).release();
     }
 
+    @Test
+    public void testNullINsdManagerCallback() {
+        final NsdService service = new NsdService(mContext, mHandler, CLEANUP_DELAY_MS, mDeps) {
+            @Override
+            public INsdServiceConnector connect(INsdManagerCallback baseCb,
+                    boolean runNewMdnsBackend) {
+                // Pass null INsdManagerCallback
+                return super.connect(null /* cb */, runNewMdnsBackend);
+            }
+        };
+
+        assertThrows(IllegalArgumentException.class, () -> new NsdManager(mContext, service));
+    }
+
     private void waitForIdle() {
         HandlerUtils.waitForIdle(mHandler, TIMEOUT_MS);
     }