Delete volume is empty (#4561)

* use onlyEmpty for deleteVolume
https://github.com/seaweedfs/seaweedfs/issues/4559

* fix IsEmpty

* fix test

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

weed/storage/volume.go

@@ -332,3 +332,6 @@ func (v *Volume) IsReadOnly() bool {
 	defer v.noWriteLock.RUnlock()
 	return v.noWriteOrDelete || v.noWriteCanDelete || v.location.isDiskSpaceLow
 }
+func (v *Volume) IsEmpty() bool {
+	return v.ContentSize() == 0
+}