Bluetooth: Mesh: Initialize UUID in shell's mod_pub_set

PR #35774 introduced a uuid field in the bt_mesh_cfg_mod_pub structure.
The shell does not initialize this pointer before passing it to the
access layer. Add a line to initialize this pointer.

Fixes #38016.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>