D8380: Modernize: use nullptr

[Laurent Montel]

mlaurent added a reviewer: sebas.

REPOSITORY
  R104 KScreen

REVISION DETAIL
  https://phabricator.kde.org/D8380

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