Modernize the code: use override and nullptr
Summary:
Used run-clang-tidy.py: https://www.kdab.com/clang-tidy-part-1-modernize-source-code-using-c11c14/
GIT_SILENT
Test Plan: Build
Reviewers: piggz
Reviewed By: piggz
Tags: KReport
Maniphest Tasks: T5728
Differential Revision: https://phabricator.kde.org/D5629