Yes it is. And no it should not have. An empty screenwash IS an instant MOT failure, yes, not only that but its one of THE most common failures!. It is also your legal responsability to make sure you have enough screenwash in your vehicle for your journey and you can face penalties for having no screenwash. Yes this is extraordinarily unlikely, but it is the law. Quick google check leads me to this...
http://www.dailymail.co.uk/news/article-2583943/Easy-fix-MoT-failures-cost-thousands-Poorly-placed-stickers-windscreens-screen-wash-ridiculous-reasons-cars-failing.html
Also, as of 2012 any warning light can lead to an MOT failure, this was a European initiative brought in Europe wide. It has taken a while for the legal side of things to take effect in the UK, but i believe they took effect in 2013/14. My vehicle failed its MOT for a air bag warning light, despite the airbag being disconnected and no airbag present. My truck failed its plating for an Engine Oil Light warning, and that was a dodgy sensor.
There are many MOT stations that pass vehicles that technically should not pass, I have had cars pass with bald tyres, Motorcycles with straight through noisy exhausts and semi slick race tyres. This doesnt mean they arent MOT failures, just not so great testing stations.