param: Add null checks for upcoming str_list_make changes

param: Add null checks for upcoming str_list_make changes

In changing str_list_make to str_list_make_v3, the list can be NULL.
These are some additional checks to try to avoid any problems.

Where lists are dealt with, they typically check both if the list is
empty or the list is NULL.

Change-Id: I9012c31dbd9832ce877728bcb3346616ba64c4c5
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Nadezhda Ivanova <nivanova@samba.org>