Don't set GAE location unless property available

buildSrc/src/main/groovy/gae/GaePlugin.groovy

@@ -7,10 +7,10 @@ class GaePlugin implements Plugin<Project> {
     public void apply(Project project) {
         if (!project.hasProperty('appEngineSdkRoot')) {
             println "'appEngineSdkRoot' must be set in gradle.properties"
+        } else {
+            System.setProperty('appengine.sdk.root', project.property('appEngineSdkRoot'))
         }
 
-        System.setProperty('appengine.sdk.root', project.property('appEngineSdkRoot'))
-
         File explodedWar = new File(project.buildDir, "gae-exploded")
 
         project.task('gaeDeploy') << {