Summary plot breaks if no students are assigned
When the summary plot is requested for the overview page it will return 500 when there are no assigned students yet.
The following error is thrown:
[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] 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/summary_plot.py", line 100, in get
[PYTHON] data[0], data[1], 0.5, data[2], color=cm(norm(data[3])), align='center',
[PYTHON] IndexError: index 0 is out of bounds for axis 0 with size 0
This is low priority since the summary plot will be replaced with client side plots in !174 (closed).