Fix: Use StorageReference::is_valid() in assertions

Changed FIREBASE_ASSERT_RETURN to use is_valid() instead of
internal_->is_valid() in StorageReference::ListAll() and
StorageReference::List() methods.

This corrects the compilation error where 'is_valid' was not a member
of the internal PIMPL class. The assertion now correctly checks the
validity of the public StorageReference object before proceeding.

Author: google-labs-jules[bot]

storage/src/common/storage_reference.cc

@@ -257,12 +257,12 @@ Future<Metadata> StorageReference::PutFileLastResult() {
 bool StorageReference::is_valid() const { return internal_ != nullptr; }
 
 Future<ListResult> StorageReference::ListAll() {
-  FIREBASE_ASSERT_RETURN(Future<ListResult>(), internal_->is_valid());
+  FIREBASE_ASSERT_RETURN(Future<ListResult>(), is_valid());
   return internal_->ListAll();
 }
 
 Future<ListResult> StorageReference::List(const char* page_token) {
-  FIREBASE_ASSERT_RETURN(Future<ListResult>(), internal_->is_valid());
+  FIREBASE_ASSERT_RETURN(Future<ListResult>(), is_valid());
   return internal_->List(page_token);
 }