QDateTime: use default ctor to create invalid object

Don't perform some internal init functions.

Change-Id: I9986e7a8adab35499aea804d1019012547aefd5d
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qdatetime.cpp

@@ -4699,7 +4699,7 @@ QDateTime QDateTime::fromString(const QString &string, const QString &format)
     Q_UNUSED(string);
     Q_UNUSED(format);
 #endif
-    return QDateTime(QDate(), QTime(-1, -1, -1));
+    return QDateTime();
 }
 
 #endif // QT_NO_DATESTRING