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)

core/src/main/java/hudson/model/Computer.java

@@ -457,7 +457,7 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
      * Returns {@link Node#getNodeName() the name of the node}.
      */
     public @Nonnull String getName() {
-        return nodeName;
+        return nodeName != null ? nodeName : "";
     }
 
     /**

core/src/main/java/hudson/model/Node.java

@@ -117,7 +117,7 @@ public abstract class Node extends AbstractModelObject implements Reconfigurable
      *      "" if this is master
      */
     @Exported(visibility=999)
-    public abstract String getNodeName();
+    public abstract @Nonnull String getNodeName();
 
     /**
      * When the user clones a {@link Node}, Hudson uses this method to change the node name right after

core/src/main/java/hudson/model/Slave.java

@@ -56,8 +56,8 @@ import java.util.Set;
 
 import javax.servlet.ServletException;
 
-import hudson.util.TimeUnit2;
 import javax.annotation.CheckForNull;
+import javax.annotation.Nonnull;
 import jenkins.model.Jenkins;
 import jenkins.slaves.WorkspaceLocator;
 
@@ -149,7 +149,7 @@ public abstract class Slave extends Node implements Serializable {
     	this(name, nodeDescription, remoteFS, numExecutors, mode, labelString, launcher, retentionStrategy, new ArrayList());
     }
     
-    public Slave(String name, String nodeDescription, String remoteFS, int numExecutors,
+    public Slave(@Nonnull String name, String nodeDescription, String remoteFS, int numExecutors,
                  Mode mode, String labelString, ComputerLauncher launcher, RetentionStrategy retentionStrategy, List<? extends NodeProperty<?>> nodeProperties) throws FormException, IOException {
         this.name = name;
         this.description = nodeDescription;

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -170,7 +170,13 @@ public class SlaveComputer extends Computer {
 
     @Override
     public Slave getNode() {
-        return (Slave)super.getNode();
+        Node node = super.getNode();
+        if (node == null || node instanceof Slave) {
+            return (Slave)node;
+        } else {
+            logger.log(Level.WARNING, "found an unexpected kind of node {0} from {1} with nodeName={2}", new Object[] {node, this, nodeName});
+            return null;
+        }
     }
 
     @Override