Error while exporting when there are no students
When there are students with submissions, exporting throws the following error:
[PYTHON] Traceback (most recent call last):
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
[PYTHON] return self.wsgi_app(environ, start_response)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
[PYTHON] response = self.handle_exception(e)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask_restful/__init__.py", line 269, in error_router
[PYTHON] return original_handler(e)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
[PYTHON] reraise(exc_type, exc_value, tb)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
[PYTHON] raise value
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
[PYTHON] response = self.full_dispatch_request()
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
[PYTHON] rv = self.handle_user_exception(e)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask_restful/__init__.py", line 269, in error_router
[PYTHON] return original_handler(e)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
[PYTHON] reraise(exc_type, exc_value, tb)
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
[PYTHON] raise value
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
[PYTHON] rv = self.dispatch_request()
[PYTHON] File "/home/hugo/miniconda3/envs/zesje-dev/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
[PYTHON] return self.view_functions[rule.endpoint](**req.view_args)
[PYTHON] File "/home/hugo/Documents/zesje/zesje/api/export.py", line 38, in exam
[PYTHON] data = full_exam_data(exam_id)
[PYTHON] File "/home/hugo/Documents/zesje/zesje/statistics.py", line 71, in full_exam_data
[PYTHON] for problem in exam.problems.order_by(Problem.id):
[PYTHON] AttributeError: 'InstrumentedList' object has no attribute 'order_by'