There is a way in Javascript which has been posted various times in the forum, but you probably will have to check every single question on the form via Javascript, which can mean quite a lot of coding.
Just out of curiosity: why don't you make all questions mandatory?
Help us to help you!
Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.