Sept 13 (Reuters) - The FBI informed Twitter Inc (TWTR.N) of at least one Chinese agent working at the company, U.S. Senator Chuck Grassley said during a Senate hearing on Tuesday where a whistleblower testified, raising new concerns about foreign meddling at the influential social media platform. Peiter "Mudge" Zatko, a famed hacker who served as Twitter's head of security until his firing in January, said some Twitter employees were concerned the Chinese government would be able to collect data on the company's users. Twitter has come under fire previously for lax security, most notably in 2020 when teenage hackers...