Fix error when source absent from GTF exporter
Fix error when source absent from GTF exporter

jhuttner/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,jhuttner/dx-toolkit