Fix issue 1319: Error on the preview, Android 4.1.2

src/com/owncloud/android/operations/UploadFileOperation.java

@@ -332,9 +332,9 @@ public class UploadFileOperation extends RemoteOperation {
             // location in the ownCloud local folder
             if (result.isSuccess()) {
                 if (mLocalBehaviour == FileUploader.LOCAL_BEHAVIOUR_FORGET) {
-                    mFile.setStoragePath(null);
+                    mFile.setStoragePath("");
                 } else if (mLocalBehaviour == FileUploader.LOCAL_BEHAVIOUR_REMOVE){
-                    mFile.setStoragePath(null);
+                    mFile.setStoragePath("");
                     originalFile.delete();
                 } else {
                     mFile.setStoragePath(expectedPath);