BELGRADE, Serbia — Novak Djokovic returned home Monday after being thwarted from defending his Australian Open title only to face a new predicament: He could be barred from the French Open this year, too, if he’s still not vaccinated against COVID-19. A plane carrying the No. 1-ranked player touched down in his native Serbia, closing at least the first chapter in a dizzying drama that has resonance in the world of elite sports, Australia’s pandemic politics and the polarized debate over the coronavirus shots. A handful of fans waving the Serbian flag greeted him at Belgrade’s airport.