When PHH bought CUNA Mutual Mortgage and most of its 175 employees, it left a void in the small bank and credit union market. A former CUNA executive intends to enter that space by forming a new lending operation targeting credit unions. The company formed from CUNA’s ashes is Greystone Residential Funding. Dan Rotert, Greystone president and CEO, was CUNA’s COO, but he left the company in February, 10 months before PHH acquired CUNA.…