Use undefined year value

Activate/deactivate the time selector and see the time is removed correctly  now 

[test](http://mf-geoadmin3.dev.bgdi.ch/fix_time/?lang=fr&topic=ech&bgLayer=ch.swisstopo.pixelkarte-farbe&X=206014.89&Y=654676.59&zoom=1&layers_timestamp=20121231&time=2012&layers=ch.swisstopo.zeitreihen)