Hello, Welead,
In our opinion, it is really strange that customers could click search without selecting any of the choices. For this time you may reach the result you want to have only with customizations written by your own in child-theme/js/script.js or contact to services that provides such options.
We don’t use conditions for “Search” button of that filter and no customers asked us about that solution yet but If your request on our https://www.8theme.com/taskboard/ will become popular among others we will move it to next step of global implementation or consider its possibilities of implementation.
Kind Regards,
8theme team