The office slump stands out in a city that’s finally found its footing after years of pandemic clampdowns and strict security measures from Beijing that sparked an exodus of families. Retail sales are picking up, restaurants are packed and punters are jamming the Happy Valley racetrack once again. Yet a record 1.2 million square metres of office space sits empty. The overall grade A vacancy rate was almost 15 per cent in April, data from Colliers International show. That’s more than three times higher than in 2019, and tops Manhattan’s rate of 12.5 per cent, and the 4.6 per cent…