By Steve Holland and Jeff Mason WASHINGTON (Reuters) -President Joe Biden met with the leaders of Finland and Sweden at the White House on Thursday to offer robust U.S. support for their applications to join NATO, while Turkey threatened to block the Nordic nations from becoming members of the alliance. Biden, who has rallied the West to stand up to Russia after its invasion of Ukraine, joined Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinistö in a sunny White House Rose Garden bedecked with their countries’ flags in a show of unity and support. “Finland and Sweden…