vteratipally: Add a check if the metric is nil so that collector doesn't collect metrics.

JobBuild
587
07-29 19:55
pull-npd-build 1432734325895335936
pull-npd-e2e-kubernetes-gce-gci 1432734326008582144 1420835437542379520
pull-npd-e2e-kubernetes-gce-gci-custom-flags 1432734326050525184 1420835437571739648
pull-npd-e2e-kubernetes-gce-ubuntu 1432734326084079616 1420835437605294080
pull-npd-e2e-kubernetes-gce-ubuntu-custom-flags 1432734326117634048 1420835437647237120
pull-npd-e2e-node 1432734325966639104
pull-npd-e2e-test 1432734326151188480
pull-npd-test 1432734325928890368