fix(firebase_core): Fix FirebaseException error code bug by making default value: "unknown". (#6897)

Co-authored-by: Russell Wheatley <russellwheatley85@gmail.com>

Author: hyungtaecf

packages/firebase_core/firebase_core_platform_interface/lib/src/firebase_exception.dart

@@ -30,9 +30,10 @@ class FirebaseException implements Exception {
   FirebaseException({
     required this.plugin,
     this.message,
-    this.code = 'unknown',
+    String? code,
     this.stackTrace,
-  });
+    // ignore: unnecessary_this
+  }) : this.code = code ?? 'unknown';
 
   /// The plugin the exception is for.
   ///