Better null safety.

In 1.509.2, probably due to a bad Groovy script:
java.lang.ClassCastException: hudson.model.Hudson cannot be cast to hudson.model.Slave
	at hudson.slaves.SlaveComputer.getNode(SlaveComputer.java:176)
	at hudson.slaves.SlaveComputer.getNode(SlaveComputer.java:92)
	at jenkins.model.Jenkins$9.compare(Jenkins.java:1547)
	at jenkins.model.Jenkins$9.compare(Jenkins.java:1543)
	at java.util.Arrays.mergeSort(Arrays.java:1270)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.mergeSort(Arrays.java:1282)
	at java.util.Arrays.sort(Arrays.java:1210)
	at jenkins.model.Jenkins.getComputers(Jenkins.java:1543)