The mortgage-backed securities sector looks to be in solid shape for the coming year, according to Wall Street analysts who say that mortgage spreads have held stubbornly tight and look more likely to tighten further than widen in the foreseeable future. “Another year-end phenomenon that investors have gotten used to over the years is the December widening in structured credit spreads. Funny thing is, it never happened this year,” said Gyan Sinha, an analyst…