VcsBase: Consistently use nullptr

Fixed by clang-tidy modernize-use-nullptr.

Change-Id: I496d3febae9c445dc96490582a5d4ae86fec71ee
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/baseannotationhighlighter.h

@@ -40,7 +40,7 @@ public:
     typedef  QSet<QString> ChangeNumbers;
 
     explicit BaseAnnotationHighlighter(const ChangeNumbers &changeNumbers,
-                                       QTextDocument *document = 0);
+                                       QTextDocument *document = nullptr);
     ~BaseAnnotationHighlighter() override;
 
     void setChangeNumbers(const ChangeNumbers &changeNumbers);