D23618: Use nullptr everywhere

[Vlad Zahorodnii]

zzag added a dependency: D23948: Don't initialize QFlags<T> with 0 value.

REPOSITORY
  R108 KWin

BRANCH
  use-nullptr-everywhere

REVISION DETAIL
  https://phabricator.kde.org/D23618

To: zzag, #kwin, davidedmundson
Cc: romangg, kwin, LeGast00n, The-Feren-OS-Dev, sbergeron, jraleigh, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart
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