Don't initialize QFlags<T> with 0 value

Summary:
clang-tidy has a check that converts all usages of null pointer literals
to nullptr keyword. However, there's a small issue related to QFlags<T>.

Apparently, QFlags<T> has a constructor that takes a pointer and if you
pass 0 to it, clang-tidy will replace 0 with nullptr, e.g.

NET::States(0) -> NET::States(nullptr)

Even though passing nullptr is totally correct, it looks very weird.

Test Plan: Complies.

Reviewers: KWin, gladhorn

Reviewed By: KWin, gladhorn

Subscribers: gladhorn, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23948