GearmanProxy: please spotbugs NPE for getComputer("")

The Jenkins getComputer() method can return `null` when the requested
name does not match any Computer. There is always an unamed Computer
which is the built-in node and the method would never return `null` in
that case.

Addresses spotbugs NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE

src/main/java/hudson/plugins/gearman/GearmanProxy.java

@@ -71,18 +71,19 @@ private GearmanProxy() {
         // query Jenkins for built-in name
         try {
             builtInNode = Jenkins.get().getComputer("");
-            hostname = builtInNode.getHostName();
+            if (builtInNode != null) {
+                hostname = builtInNode.getHostName();
+            } else {
+                // Built-in node may not be enabled so get builtInName from system
+                try {
+                    hostname = java.net.InetAddress.getLocalHost().getHostName();
+                } catch (UnknownHostException e) {
+                    logger.warn("Exception while getting built-in node hostname", e);
+                }
+            }
         } catch (Exception e) {
             logger.warn("Exception while getting hostname", e);
         }
-        // Built-in node may not be enabled so get builtInName from system
-        if (builtInNode == null) {
-            try {
-                hostname = java.net.InetAddress.getLocalHost().getHostName();
-            } catch (UnknownHostException e) {
-                logger.warn("Exception while getting hostname", e);
-            }
-        }
 
         builtInName = hostname;
     }