libpfctl: use snl_f_p_empty instead of declaring own empty array
Just as we did in the kernel in e9255dafa1ef0c635385aee78ead3a0050b19cca
Suggested by: glebius
Reviewed by: glebius, melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D48460