Import task: Handle exercises without tests / model solution files

[MrSerth]

When importing a ProFormA task that contains a test but without a test file or alternatively a model solution without a model solution file, CodeOcean throws an error.

Most likely, the error is caused through these two lines:

codeocean/app/services/proforma_service/convert_task_to_exercise.rb

Line 68 in 1e08f2c

task_files.delete(test_object.files.first.id).tap do |file|

codeocean/app/services/proforma_service/convert_task_to_exercise.rb

Line 79 in 1e08f2c

task_files.delete(model_solution_object.files.first.id).tap do |file|

Here, we are accessing the ID of the first file for both, but don't handle the case where no file exists.