Next step for use-nullptr

Summary:
The patch continues use-nullptr migration with changes for some parts of code which was not fixed by clang-tidy.

Unfortunately it's not possible now to enable "-Wzero-as-null-pointer-constant" flag without adding many ugly fixes to code. It causes by warnings/errors produced by this flag for 3rd-party, non-KDevelop code such a KF5, Qt which still use old-style zeroes instead nullptr.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3092