CppTools: Consistently use nullptr

Fixed by clang-tidy modernize-use-nullptr.

Change-Id: I951627ba3543faa357f67ee76527518290efe3f1
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/clangdiagnosticconfigswidget.h

@@ -57,7 +57,7 @@ public:
     explicit ClangDiagnosticConfigsWidget(
             const ClangDiagnosticConfigsModel &diagnosticConfigsModel = ClangDiagnosticConfigsModel(),
             const Core::Id &configToSelect = Core::Id(),
-            QWidget *parent = 0);
+            QWidget *parent = nullptr);
     ~ClangDiagnosticConfigsWidget() override;
 
     Core::Id currentConfigId() const;