Bug in created JDL when specifying a single outputfile

In the current version the logic for creating the "transfer_output_file" arguments in the jdl based on the specified outputfiles is flawed. The current version works fine when none or more then one outputfiles are provided. However for a single outputfile an additional empty "transfer_output_files" option is written in the jdl. In this pull request the logic for declaring the output file in the jdl is adapted to the same logic as used for the input files.