QTextObject: One more 0 to nullptr conversion

Change-Id: I61446afa882304400d3ae8045e4f17bb7a020600
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/gui/text/qtextobject.cpp

@@ -1322,7 +1322,7 @@ QTextList *QTextBlock::textList() const
 QTextBlockUserData *QTextBlock::userData() const
 {
     if (!p || !n)
-        return 0;
+        return nullptr;
 
     const QTextBlockData *b = p->blockMap().fragment(n);
     return b->userData;