QString::null is depriated as of QT 5.9. Use QString() instead. Non-breaking...

QString::null is depriated as of QT 5.9. Use QString() instead. Non-breaking change. Tecnically the second parameter iof KHelpClient::invokeHelp isn't even needed as will defualt to QString(). Leaving it for clarity sake.
Signed-off-by: Michael Reeves <reporter123@users.noreply.github.com>

src/optiondialog.cpp

@@ -603,7 +603,7 @@ void OptionDialog::buttonClicked( QAbstractButton *button ) {
 }
 
 void OptionDialog::helpRequested() {
-    KHelpClient::invokeHelp( QStringLiteral( "kdiff3/index.html" ), QString::null );
+    KHelpClient::invokeHelp( QStringLiteral( "kdiff3/index.html" ), QString());
 }
 
 OptionDialog::~OptionDialog( void ) {