Fix proposal for NPE in ComputerHistoryListener.onConfigurationChange
method. Our jenkins faced some restart issue with the following
stacktrace. So better to ensure the nodes list is defined on the first
run.

hudson.util.HudsonFailedToLoad: java.lang.NullPointerException
  at hudson.WebAppMain$3.run(WebAppMain.java:234)
Caused by: java.lang.NullPointerException
  at
hudson.plugins.jobConfigHistory.ComputerHistoryListener.onConfigurationChange(ComputerHistoryListener.java:29)
  at
hudson.model.AbstractCIBase.updateComputerList(AbstractCIBase.java:195)
  at jenkins.model.Jenkins.updateComputerList(Jenkins.java:1220)
  at jenkins.model.Jenkins.<init>(Jenkins.java:852)
  at hudson.model.Hudson.<init>(Hudson.java:82)
  at hudson.model.Hudson.<init>(Hudson.java:78)
  at hudson.WebAppMain$3.run(WebAppMain.java:222)
Merge pull request #39 from williambernardet/master

NPE on ComputerHistoryListener.onConfigurationChange