D8309: Modernize: Use nullptr where possible

[Kevin Funk]

kfunk added a reviewer: sitter.

REPOSITORY
  R871 DrKonqi

REVISION DETAIL
  https://phabricator.kde.org/D8309

To: kfunk, sitter
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart