morse_desc_init_user(): If one of the function get_* is NULL, we switch back...

morse_desc_init_user(): If one of the function get_* is NULL, we switch back to the default, like in morse_desc_init()

This allows to change only 1 or 2 of the 3 functions, and keep the other
unchanged. For example, in OOC, only the 1st is modified (to always return NULL).