Allow test suites without tests

[pdawyndt]

TESTed currently does not allow to setup a test suite that does not perform any test. The tabs/units array cannot be empty (None), and a tab/unit cannot have an empty array of testcases. If this were allowed, an empty test suite would allow to submit files that are checked for compilation errors (maybe added as top-level message or in a Compilation tab as in the Python judge) , and add linting messages in the Code tab, but would not have any extra tabs with testcases.

Use cases:

• use for Time2Code exercises that currently have no tests (because of limitations in TESTed that are currently under implementation by @niknetniko); for now I used the Python judge instead to implement this behaviour
• use for exams where not feedback is given to students during the exam; testcases could then be added after the exam and submissions could be rejudged (although @bmesuere also suggested to allow setting the visibilty of feedback in Dodona, instead of having it harcoded by a judge)