The word 'bare' in this meaning means empty and without the usual contents.
Here are some more examples:
'The trees in my garden look really bare in winter' (empty, without the usual leaves)
'The room looks really bare, I need some furniture' (empty, without furniture, which you expect to see in a room)
So in the sentence it means that the streets were nearly empty of people shopping.
If something is done at the 'last-minute', it means it is done just before a deadline.
e.g. 'I made some last-minute corrections to my essay before I handed it in'
In this way, then, the term 'last-minute shoppers' can mean 2 things:
- People shopping really late before the shops close
- People shopping really late before a big festival such as Christmas
Hope this is useful! Let me know if you have any questions