By default your DSLR or Bridge Camera is probably set to allow you to shoot with your camera even if you don’t have a card in the slot. This is dangerous as you could quite easily leave home, shoot all day only to find when you get home there is nothing to download as you didn’t have a card. As Homer Simpson moments go this would be a doh! with a capital D!!
What you want to do is to disable this function so that if you try to take a shot without a card you get a warning message
If you shoot with Canon it is in the red menus but if you can’t find it please give me a shout