For robustness, pass on the null to Jelly rather than failing page rendering.

Jenkins.getComputers may be called frequently; do not perform an expensive locale-sensitive sort each time.

(Also relevant to JENKINS-22395 diagnostics.)

Start cron at zero seconds after the minute, and ensure the internal calendar is set to zero seconds.  Allows for more predictable behavior.

[FIXED JENKINS-22672] Make /threadDump easier to use.