param: pre-emptively correct use of str_list_make_v3 in lib/param

param: pre-emptively correct use of str_list_make_v3 in lib/param

lib/param uses str_list_make, while, s3 uses str_list_make_v3. These
differ slightly and should be made the same.

Notably it returns NULL when given a null or empty string

Also, includes the null check in s3 code to be consistent for merging
this section of code later on.

Change-Id: Ib543a5a5307e06989621ea16bfe87e315d66a7ef
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Nadezhda Ivanova <nivanova@samba.org>