If a company offers to set up a group health plan but no employees want to sign up, is that still considered an “offer of coverage” for purposes of the employer mandate, or does the plan actually have to be implemented?

It actually has to be implemented.

If it’s not implemented, what happens if there is a new employee?

Another example: what if an employee “waives” coverage in the non-existent plan because he/she has coverage through a spouse but then the spouse loses that coverage? There would be no plan for the employee and his/her spouse to special enroll into.