Don't use DataGrid for empty states (Closes #1636) (#1644)

* Don't use DataGrid for empty states (Closes #1636)

there's precedence for this behavior in `grades_semester_view.html`:
https://github.com/e-valuation/EvaP/blob/cb49c92e125bf0dcd5bdafec504702316dab9049/evap/grades/templates/grades_semester_view.html#L27



* fix same problem for grades_semester_view

* Apply suggestions from code review

Co-authored-by: Niklas Mohrin <dev@niklasmohrin.de>

Co-authored-by: Johannes Wolf <johannes-wolf@posteo.de>
Co-authored-by: Niklas Mohrin <dev@niklasmohrin.de>