Dangling symref updates do not create reflog entries

Whenever a ref update happens, a corresponding reflog entry is made. This is also true for symref updates. But for dangling symref updates, where the target ref doesn't exist. We do not create reflog entries. While the reflog mechanism doesn't allow us to store ref targets, it would still be nice to have a zero oid reflog entry to ensure we track changes in the symref.