Updating method signature

Author: mansisampat

packages/auth/src/core/auth/auth_impl.ts

@@ -675,7 +675,7 @@ export class AuthImpl implements AuthInternal, _FirebaseService {
   }
 
   onFirebaseTokenChanged(
-    nextOrObserver: NextOrObserver<FirebaseToken>,
+    nextOrObserver: NextOrObserver<FirebaseToken | null>,
     error?: ErrorFn,
     completed?: CompleteFn
   ): Unsubscribe {

packages/auth/src/model/auth.ts

@@ -82,7 +82,7 @@ export interface AuthInternal extends Auth {
   emulatorConfig: EmulatorConfig | null;
   getFirebaseAccessToken(forceRefresh?: boolean): Promise<string | null>;
   onFirebaseTokenChanged(
-    nextOrObserver: NextOrObserver<FirebaseToken>,
+    nextOrObserver: NextOrObserver<FirebaseToken | null>,
     error?: ErrorFn,
     completed?: CompleteFn
   ): Unsubscribe;