How do professional athletes get paid when they have no spectators?

TV rights. Promoters that pay professional athletes make little to nothing from the ticket sales, tickets cover the costs of the facility.

Baseball players are paid by their teams, who played to empty ball parks in 2020. Same with basketball and football. But they all were broadcast on TV and the TV rights paid the bills.

And (team-)sponsors, and season ticket holders, and personal sponsorships (think clothing, shoes, portrait rights (e-games)).

They work on a contract.