lib: `show route-map` should not print (null)

lib: `show route-map` should not print (null)

This command:
route-map FOOBAR permit 10
 set ipv6 next-hop prefer-global
 set community 5060:12345 additive
!

When you issue a `show route-map ...` command displays this:

route-map: FOOBAR Invoked: 0 (0 milliseconds total) Optimization: enabled Processed Change: false
 permit, sequence 5 Invoked 0 (0 milliseconds total)
  Match clauses:
  Set clauses:
    ipv6 next-hop prefer-global (null)
    community 5060:12345 additive
  Call clause:
  Action:
    Exit routemap

Modify the code so that it no longer displays the NULL when there
is nothing to display.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>