Avoid NullPointerException when a job has no description.
Avoid NullPointerException when a job has no description.

See http://ci.bazel.io/job/CR/job/gerrit-verifier/793/console
for a breakage example.

Change-Id: I39946bacac28efa5216d07de74be37d815bff90d

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration