BUG/MINOR: stick-table: Always call smp_fetch_src() with a valid arg list

BUG/MINOR: stick-table: Always call smp_fetch_src() with a valid arg list

The sample fetch functions must always be called with a valid argument
list. When called by hand, if there is no argument to pass, empty_arg_list must
be used.

In the stick-table code, there are some calls to smp_fetch_src() with NULL as
argument list. It is changed to use empty_arg_list instead. It is not really a
bug because smp_fetch_src() does not use the argument list. But it is an API
bug.

This patch may be backported to all stable branches as a cleanup.

(cherry picked from commit bdbd5db2a50461fceb7fb89329e73d08f90e44fd)
Signed-off-by: Willy Tarreau <w@1wt.eu>