The Senate Finance Committee is probing the Obama administration’s 2015 decision to approve the sale of a U.S. company with insight into “military applications” to the Chinese government and an investment firm run by former Vice President Joe Biden’s youngest son, Hunter Biden. Sen. Chuck Grassley (R-IA), the committee’s chairman, sent a letter to the Treasury Secretary Steve Mnuchin on Thursday requesting documents relating to the sale of Henniges, a Michigan-based automotive company, to Aviation…