fix from comment delete volume is empty (#4573)

* fix from coments
https://github.com/seaweedfs/seaweedfs/pull/4561

* fix tests

---------

Co-authored-by: Konstantin Lebedev <9497591+kmlebedev@users.noreply.github.co>

weed/storage/store.go

@@ -509,12 +509,12 @@ func (s *Store) UnmountVolume(i needle.VolumeId) error {
 
 func (s *Store) DeleteVolume(i needle.VolumeId, onlyEmpty bool) error {
 	v := s.findVolume(i)
-	if onlyEmpty && !v.IsEmpty() {
-		return fmt.Errorf("delete volume %d not empty", i)
-	}
 	if v == nil {
 		return fmt.Errorf("delete volume %d not found on disk", i)
 	}
+	if onlyEmpty && !v.IsEmpty() {
+		return fmt.Errorf("delete volume %d not empty", i)
+	}
 	message := master_pb.VolumeShortInformationMessage{
 		Id:               uint32(v.Id),
 		Collection:       v.Collection,

weed/storage/volume.go

@@ -333,5 +333,6 @@ func (v *Volume) IsReadOnly() bool {
 	return v.noWriteOrDelete || v.noWriteCanDelete || v.location.isDiskSpaceLow
 }
 func (v *Volume) IsEmpty() bool {
-	return v.ContentSize() == 0
+	size, _, _ := v.DataBackend.GetStat()
+	return size <= 8 && v.ContentSize() == 0
 }