Add TestTagsAuthKeyConvertToUserViaCLIRegister that reproduces the
exact panic from #3038: register a node with a tags-only PreAuthKey
(no user), force reauth with empty tags, then register via CLI with
a user. The mapper panics on node.Owner().Model().ID when User is nil.
The critical detail is using a tags-only PreAuthKey (User: nil). When
the key is created under a user, the node inherits the User pointer
from createAndSaveNewNode and the bug is masked.
Also add Owner() validity assertions to the existing unit test
TestTaggedNodeWithoutUserToDifferentUser to catch the nil pointer
at the unit test level.
Updates #3038
ca75e096e6