With renters flocking to Walker House’s 264 apartments and commitments from a host of retailers, L+M Development Partners is keenly focused on the third piece of its formula. The New York-based developer, alongside brokers with JLL, has stepped up efforts to market more than 50,000 square feet of available office space at the building, touting the promise of downtown Newark’s resurgence and a workplace that is like few others in New Jersey.