prevent php-warnings, closes #1807

app/controllers/admission/courseset.php

app/controllers/admission/restricted_courses.php

app/controllers/admission/rule.php

app/controllers/admission/userlist.php

app/controllers/calendar/calendar.php

app/controllers/calendar/contentbox.php

app/controllers/calendar/group.php

app/controllers/calendar/instschedule.php

app/controllers/calendar/schedule.php

app/controllers/consultation/admin.php

app/controllers/course/admission.php

app/controllers/course/basicdata.php

app/controllers/course/block_appointments.php

app/controllers/course/cancel_dates.php

app/controllers/course/dates.php

app/controllers/course/elearning.php

app/controllers/course/enrolment.php

app/controllers/course/forum/admin.php

app/controllers/course/forum/area.php

app/controllers/course/forum/index.php