Oh wow, it's a leaf curling spider. I didn't know they spin webs without leaves! This one didn't have one - is that common? Maybe it didn't find any or was working on it...
Generally they will have a leaf, or sometimes a snail shell, within their webs. Although like you said, it might have just started to build the web and yet to add a leaf. You could check again and see how the leaf situation has developed!
Rarely will you see leaf curling spiders without a leaf however