fix: null check (#10119)

Author: Yokoi-K

packages/firebase_auth/firebase_auth_platform_interface/lib/src/method_channel/method_channel_firebase_auth.dart

@@ -657,7 +657,7 @@ class MethodChannelFirebaseAuth extends FirebaseAuthPlatform {
 
           FirebaseAuthException exception = FirebaseAuthException(
             message: details != null ? details['message'] : error?['message'],
-            code: details != null ? details['code'] : 'unknown',
+            code: details?['code'] ?? 'unknown',
           );
 
           verificationFailed(exception);