Fix NullPointerException with old configs

Fixes errors like this:

```
java.lang.NullPointerException
	at au.com.rayh.XCodeBuilder.perform(XCodeBuilder.java:575)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:781)
	at hudson.model.Build$BuildExecution.build(Build.java:199)
	at hudson.model.Build$BuildExecution.doRun(Build.java:160)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:562)
	at hudson.model.Run.execute(Run.java:1665)
	at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:246)
```