b2: Fix listing root buckets with unrestricted API key

Fixes previous pull request #8978

An oversight meant that unrestricted API keys
never called b2_list_buckets,
meaning the root remote could not be listed.

The call is now made in the event there are no allowed buckets,
indicating an unrestricted API key

Fixes #9007

backend/b2/b2.go

@@ -1081,21 +1081,10 @@ type listBucketFn func(*api.Bucket) error
 func (f *Fs) listBucketsToFn(ctx context.Context, bucketName string, fn listBucketFn) error {
 	responses := make([]api.ListBucketsResponse, len(f.info.APIs.Storage.Allowed.Buckets))[:0]
 
-	for i := range f.info.APIs.Storage.Allowed.Buckets {
-		b := &f.info.APIs.Storage.Allowed.Buckets[i]
-		// Empty names indicate a bucket that no longer exists, this is non-fatal
-		// for multi-bucket API keys.
-		if b.Name == "" {
-			continue
-		}
-		// When requesting a specific bucket skip over non-matching names
-		if bucketName != "" && b.Name != bucketName {
-			continue
-		}
-
+	call := func(id string) error {
 		var account = api.ListBucketsRequest{
 			AccountID: f.info.AccountID,
-			BucketID:  b.ID,
+			BucketID:  id,
 		}
 		if bucketName != "" && account.BucketID == "" {
 			account.BucketName = f.opt.Enc.FromStandardName(bucketName)
@@ -1114,6 +1103,32 @@ func (f *Fs) listBucketsToFn(ctx context.Context, bucketName string, fn listBuck
 			return err
 		}
 		responses = append(responses, response)
+		return nil
+	}
+
+	for i := range f.info.APIs.Storage.Allowed.Buckets {
+		b := &f.info.APIs.Storage.Allowed.Buckets[i]
+		// Empty names indicate a bucket that no longer exists, this is non-fatal
+		// for multi-bucket API keys.
+		if b.Name == "" {
+			continue
+		}
+		// When requesting a specific bucket skip over non-matching names
+		if bucketName != "" && b.Name != bucketName {
+			continue
+		}
+
+		err := call(b.ID)
+		if err != nil {
+			return err
+		}
+	}
+
+	if len(f.info.APIs.Storage.Allowed.Buckets) == 0 {
+		err := call("")
+		if err != nil {
+			return err
+		}
 	}
 
 	f.bucketIDMutex.Lock()