5dd11ab5f36ce71138005 did a similar change for conf_files_list_strv().
Here we do the same for conf_files_list() and conf_files_list_nulstr().
No change for existing users. Tests are added.
va_list ap;
assert(strv);
va_list ap;
assert(strv);
va_start(ap, dir);
dirs = strv_new_ap(dir, ap);
va_start(ap, dir);
dirs = strv_new_ap(dir, ap);
_cleanup_strv_free_ char **dirs = NULL;
assert(strv);
_cleanup_strv_free_ char **dirs = NULL;
assert(strv);
dirs = strv_split_nulstr(d);
if (!dirs)
dirs = strv_split_nulstr(d);
if (!dirs)