QJsonValue: use Q_NULLPTR instead of 0 in ctor

Use Q_NULLPTR in all public headers

Task-number: QTBUG-45291
Change-Id: Ib294deb3c210a9a186448cbf9656af7a09fea2c1
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/json/qjsonvalue.h

@@ -76,7 +76,7 @@ public:
     QJsonValue(QLatin1String s);
 #ifndef QT_NO_CAST_FROM_ASCII
     inline QT_ASCII_CAST_WARN QJsonValue(const char *s)
-        : d(0), t(String) { stringDataFromQStringHelper(QString::fromUtf8(s)); }
+        : d(Q_NULLPTR), t(String) { stringDataFromQStringHelper(QString::fromUtf8(s)); }
 #endif
     QJsonValue(const QJsonArray &a);
     QJsonValue(const QJsonObject &o);