bgpd: no router bgp check candidate config

bgpd: no router bgp check candidate config

For `no router bgp` without ASN check candidate
config for default bgp instance presence to avoid
failure from checking backend db where bgp instance
may not be created.
This situation can be seen in transactional cli mode
with following config.

bharat(config)# router bgp 101
bharat(config-router)# exit
bharat(config)# no router bgp
% No BGP process is configured
bharat(config)# no router bgp
% No BGP process is configured
bharat(config)#

Signed-off-by: Chirag Shah <chirag@nvidia.com>