Exclude tasks with no status from task_state_time
Exclude tasks with no status from task_state_time

These tasks have no timestamp defined, and 0 doesn't seem like a useful substitute timestamp here (I think, at least), so maybe just skip them?

Fixes #6
ericgerling/mesos_exporter,Fra-nk/mesos_exporter,mesosphere/mesos_exporter,StephanErb/mesos_exporter