Doc: Document 'empty' operator

Thanks to Alexander Zubkov for the notification.
49 jobs from master in 17 minutes 23 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Image
passed #49082
dind allowed to fail
docker_centos-6-amd64

00:27

passed #49083
dind allowed to fail
docker_centos-7-amd64

00:29

passed #49072
dind allowed to fail
docker_debian-7-amd64

04:32

passed #49076
dind allowed to fail
docker_debian-7-i386

00:28

passed #49073
dind allowed to fail
docker_debian-8-amd64

06:00

passed #49077
dind allowed to fail
docker_debian-8-i386

00:25

passed #49074
dind allowed to fail
docker_debian-9-amd64

05:31

passed #49078
dind allowed to fail
docker_debian-9-i386

00:51

passed #49075
dind allowed to fail
docker_debian-testing-amd64

07:09

passed #49079
dind allowed to fail
docker_debian-testing-i386

00:56

passed #49080
dind allowed to fail
docker_fedora-25-amd64

04:34

passed #49081
dind allowed to fail
docker_fedora-26-amd64

05:12

passed #49084
dind allowed to fail
docker_opensuse-42_3-amd64

00:51

passed #49085
dind allowed to fail
docker_ubuntu-14_04-amd64

03:24

passed #49086
dind allowed to fail
docker_ubuntu-16_04-amd64

03:08

 
  Build
passed #49099
docker linux amd64
build-centos-6-amd64

00:55

passed #49100
docker linux amd64
build-centos-6-amd64-v6

00:56

passed #49101
docker linux amd64
build-centos-7-amd64

00:59

passed #49102
docker linux amd64
build-centos-7-amd64-v6

00:59

passed #49087
docker linux amd64
build-debian-7-amd64

01:13

passed #49091
docker linux amd64
build-debian-7-amd64-v6

01:09

passed #49109
docker linux amd64
build-debian-7-i386

01:38

passed #49110
docker linux amd64
build-debian-7-i386-v6

01:39

passed #49088
docker linux amd64
build-debian-8-amd64

01:15

passed #49092
docker linux amd64
build-debian-8-amd64-v6

01:09

passed #49111
docker linux amd64
build-debian-8-i386

01:34

passed #49112
docker linux amd64
build-debian-8-i386-v6

01:36

passed #49089
docker linux amd64
build-debian-9-amd64

01:17

passed #49093
docker linux amd64
build-debian-9-amd64-v6

01:08

passed #49113
docker linux amd64
build-debian-9-i386

01:37

passed #49114
docker linux amd64
build-debian-9-i386-v6

01:33

passed #49090
docker linux amd64
build-debian-testing-amd64

01:08

passed #49094
docker linux amd64
build-debian-testing-amd64-v6

01:32

passed #49115
docker linux amd64
build-debian-testing-i386

01:31

passed #49116
docker linux amd64
build-debian-testing-i386-v6

01:01

passed #49095
docker linux amd64
build-fedora-25-amd64

01:18

passed #49096
docker linux amd64
build-fedora-25-amd64-v6

01:18

passed #49097
docker linux amd64
build-fedora-26-amd64

01:15

passed #49098
docker linux amd64
build-fedora-26-amd64-v6

01:22

passed #49117
freebsd amd64
build-freebsd-11-amd64

07:17

passed #49118
freebsd amd64
build-freebsd-11-amd64-v6

07:10

passed #49119
freebsd i386
build-freebsd-11-i386

07:13

passed #49120
freebsd i386
build-freebsd-i386-v6

07:16

passed #49103
docker linux amd64
build-opensuse-42_3-amd64

01:07

passed #49104
docker linux amd64
build-opensuse-42_3-amd64-v6

01:02

passed #49105
docker linux amd64
build-ubuntu-14_04-amd64

01:03

passed #49106
docker linux amd64
build-ubuntu-14_04-amd64-v6

01:15

passed #49107
docker linux amd64
build-ubuntu-16_04-amd64

00:50

passed #49108
docker linux amd64
build-ubuntu-16_04-amd64-v6

01:36