bgpd: `neighbor X:X::X default-originate` complains about (null)

bgpd: `neighbor X:X::X default-originate` complains about (null)

The `neighbor X:X::X default-originate command is complaining
that:
The route-map '(null)' does not exist.

Upon inspection of the code we were passing a NULL
string to the lookup.  Testing for null gets us this:

donna.cumulusnetworks.com# conf t
donna.cumulusnetworks.com(config)# router bgp 99
donna.cumulusnetworks.com(config-router)# neighbor 2001:1::1:2 remote-as 99
donna.cumulusnetworks.com(config-router)# neighbor 2001:1::1:2 default-originate
donna.cumulusnetworks.com(config-router)# end
donna.cumulusnetworks.com# show run
Building configuration...

Current configuration:
!
frr version 7.2-dev
frr defaults datacenter
hostname donna.cumulusnetworks.com
log stdout
no ipv6 forwarding
!
ip route 4.5.6.7/32 10.50.11.4
!
router bgp 99
 neighbor 2001:1::1:2 remote-as 99
 !
 address-family ipv4 unicast
  neighbor 2001:1::1:2 default-originate

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