adjustments due to phpstan report, fixes #2257

lib/classes/ContentBar.php

lib/classes/CoursewarePDFCertificate.php

lib/classes/JsonApi/Routes/Courseware/UnitsCopy.php

lib/classes/JsonApi/Routes/Courseware/UnitsCreate.php

lib/classes/JsonApi/Routes/Courseware/UnitsUpdate.php

lib/classes/JsonApi/Routes/Files/FileRefsContentShow.php

lib/classes/Virusscanner.php

lib/cronjobs/remind_oer_upload.class.php

lib/cronjobs/send_mail_notifications.class.php

lib/models/QuestionnaireInfo.php

lib/models/User.class.php