Homebuilders may get a much-needed boost this year from northern Nevada's improving residential real estate market, which is signaling that it's time to build, particularly at the low end.But stubbornly high office, industrial and retail vacancy rates likely mean no commercial construction anytime soon. "There is a lack of inventory and an opportunity for builders, particularly at the entry level," said Helen Graham, president of the Reno/Sparks Association of Realtors and an agent with Keller Williams Group One.Graham was among speakers at the Builders Association of Northern Nevada Forecast 2013 last week in Reno. Graham said first-time homebuyers are being …