Sydney Sweeney is keeping her knockers safe from getting knocked out in a shocking transformation for her gritty new boxing role. The Euphoria star is painfully taping down her busty chest while embodying female boxer Christy Martin in her new biopic, RadarOnline.com can reveal. A source told RadarOnline.com that while Sweeney usually flaunts her famous figure in other roles, she’s enduring the painful daily grind of strapping them down for filming. They said: “Sydney throws everything into what she sees as the art of acting, and has gone to extreme lengths to keep herself in shape for all her…