make missing Key_Back and Key_Forward menu glyphs return 0  on mac(cocoa)
make missing Key_Back and Key_Forward menu glyphs return 0  on mac(cocoa)

This is an addition to
commit 5651d2547261ccecbf50f8141c634f59c0adc00d to make it obvious that
in cocoa nothing is returned.

Task-number: QTBUG-4873
Reviewed-by: MortenS

KDE/qt,radekp/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,KDE/qt,pruiz/wkhtmltopdf-qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,radekp/qt,KDE/qt,RLovelett/qt,KDE/qt,pruiz/wkhtmltopdf-qt,radekp/qt,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,KDE/qt,radekp/qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,igor-sfdc/qt-wk,RLovelett/qt,RLovelett/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,KDE/qt,KDE/qt,RLovelett/qt,igor-sfdc/qt-wk,radekp/qt,radekp/qt,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,igor-sfdc/qt-wk