fixed NullPointerException in ProjectUploadService

Testruns:
[branch test](http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/1038/)
[single UI](http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/700/)

This will fix errors like [there](http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/1031/artifact/logcat.txt):
```
05-17 10:14:16.410 E/AndroidRuntime( 4794): FATAL EXCEPTION: IntentService[ProjectUploadService]
05-17 10:14:16.410 E/AndroidRuntime( 4794): java.lang.NullPointerException
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at java.io.File.fixSlashes(File.java:205)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at java.io.File.init(File.java:189)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at java.io.File.<init>(File.java:139)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at org.catrobat.catroid.transfers.ProjectUploadService.onHandleIntent(ProjectUploadService.java:86)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at android.app.IntentService$ServiceHandler.handleMessage(IntentService.java:59)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at android.os.Handler.dispatchMessage(Handler.java:99)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at android.os.Looper.loop(Looper.java:130)
05-17 10:14:16.410 E/AndroidRuntime( 4794): 	at android.os.HandlerThread.run(HandlerThread.java:60)