lxd/instance/post: Prevent moving instance to invalid (or empty) name
lxd/instance/post: Prevent moving instance to invalid (or empty) name

It was possible via the API to move an instance to a different cluster member using
an invalid (or empty) instance name.

Reported from https://discuss.linuxcontainers.org/t/container-name-is-set-to-an-empty-string-after-move/15566

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd