Core: Consistently use nullptr

Fixed by clang-tidy modernize-use-nullptr.

Change-Id: I62a9388ab873410555f45a8a6f9b0149467ddbaa
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/progressmanager/progressview.h

@@ -43,7 +43,7 @@ class ProgressView : public QWidget
     Q_OBJECT
 
 public:
-    ProgressView(QWidget *parent = 0);
+    ProgressView(QWidget *parent = nullptr);
     ~ProgressView() override;
 
     void addProgressWidget(QWidget *widget);