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/idocument.h

@@ -79,7 +79,7 @@ public:
         FlagIgnore
     };
 
-    IDocument(QObject *parent = 0);
+    IDocument(QObject *parent = nullptr);
     ~IDocument() override;
 
     void setId(Id id);