Try this - even though it's designed to work with networks that DO have a password:
1. Turn off WiFi
2. Turn on WiFi
3. Select "Other" - not the network you want to join
4. Manually type in the name of the network you want to join
5. Select the password type (see what works) and then hit enter since you don't have a password
A version of this works for password protected networks. I don't know whether it will work in this situation