Filter: Handle undefined BGP paths as empty

The same is already done for clists. Also fixes defined() to work
properly for paths and clists.
32 jobs for <span class="ref-name">int-new</span> in 5 minutes and 51 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Image
passed #94133
dind allowed to fail
docker_centos-6-amd64

00:00:24

passed #94134
dind allowed to fail
docker_centos-7-amd64

00:00:23

passed #94123
dind allowed to fail
docker_debian-7-amd64

00:00:23

passed #94127
dind allowed to fail
docker_debian-7-i386

00:00:23

passed #94124
dind allowed to fail
docker_debian-8-amd64

00:00:23

passed #94128
dind allowed to fail
docker_debian-8-i386

00:00:22

passed #94125
dind allowed to fail
docker_debian-9-amd64

00:00:27

passed #94129
dind allowed to fail
docker_debian-9-i386

00:00:21

passed #94126
dind allowed to fail
docker_debian-testing-amd64

00:00:24

passed #94130
dind allowed to fail
docker_debian-testing-i386

00:00:20

passed #94131
dind allowed to fail
docker_fedora-25-amd64

00:00:20

passed #94132
dind allowed to fail
docker_fedora-26-amd64

00:00:21

passed #94135
dind allowed to fail
docker_opensuse-42_3-amd64

00:00:25

passed #94136
dind allowed to fail
docker_ubuntu-14_04-amd64

00:00:25

passed #94137
dind allowed to fail
docker_ubuntu-16_04-amd64

00:00:20

 
  Build
passed #94144
docker linux amd64
build-centos-6-amd64

00:01:10

passed #94145
docker linux amd64
build-centos-7-amd64

00:01:07

passed #94138
docker linux amd64
build-debian-7-amd64

00:01:10

passed #94149
docker linux amd64
build-debian-7-i386

00:00:42

passed #94139
docker linux amd64
build-debian-8-amd64

00:01:21

passed #94150
docker linux amd64
build-debian-8-i386

00:00:45

passed #94140
docker linux amd64
build-debian-9-amd64

00:01:28

passed #94151
docker linux amd64
build-debian-9-i386

00:00:47

passed #94141
docker linux amd64
build-debian-testing-amd64

00:01:32

passed #94152
docker linux amd64
build-debian-testing-i386

00:00:47

passed #94142
docker linux amd64
build-fedora-25-amd64

00:01:27

passed #94143
docker linux amd64
build-fedora-26-amd64

00:01:20

passed #94153
freebsd amd64
build-freebsd-11-amd64

00:04:17

passed #94154
freebsd i386
build-freebsd-11-i386

00:04:20

passed #94146
docker linux amd64
build-opensuse-42_3-amd64

00:01:09

passed #94147
docker linux amd64
build-ubuntu-14_04-amd64

00:01:16

passed #94148
docker linux amd64
build-ubuntu-16_04-amd64

00:00:40