The Group of 20 leading economies struck a hopeful tone on the outlook for global growth on Friday, even as officials fretted that Athens’ inability to strike a deal with its lenders could upset Europe’s tentative recovery. In a communique after a two-day meeting, G20 finance ministers and central bankers welcomed brighter economic signs in rich nations, but lamented weakness in emerging markets. “Risks to the global economy are more balanced since we last met,” the finance officials said.