policy: fix bug of rpki validation condition
policy: fix bug of rpki validation condition

when rpki validation condition is not configured ( c == "" ), don't
create an instance for validation.

this bug was breaking all policy mechanism when rpki server is configured.

Signed-off-by: Wataru Ishida <15703620d0b11e835ab4f34ebf88f2f40cce783e@lab.ntt.co.jp>

j-kato/gobgp,fujita/gobgp,j-kato/gobgp,fujita/gobgp,osrg/gobgp,Dipsingh/gobgp,ishidawataru/gobgp,a16/gobgp,tamihiro/gobgp,Dipsingh/gobgp,a16/gobgp,Dipsingh/gobgp,fujita/gobgp,tamihiro/gobgp,osrg/gobgp,tamihiro/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp,ishidawataru/gobgp,a16/gobgp,ishidawataru/gobgp,osrg/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp