prevent php8 warnings, fixes #4925

app/controllers/course/forum/index.php

app/controllers/course/ilias_interface.php

app/controllers/course/members.php

app/controllers/new_password.php

app/controllers/oer/market.php

app/views/calendar/date/_add_edit_form.php

app/views/file/add_from_library.php

lib/classes/JsonApi/Routes/Tree/CourseInfoOfTreeNode.php

lib/classes/PluginAdministration.php

lib/classes/StudipCoreFormat.php

lib/classes/coursewizardsteps/LVGroupsWizardStep.php

lib/classes/forms/FileInput.php

lib/visual.inc.php

templates/forms/file_input.php