Interim Starbucks chief Howard Schultz faces a growing unionization push at US stores as the chain reported a modest increase in quarterly profits – Copyright AFP/File Saeed KHAN Robust sales in North America were offset by weakness in China as Starbucks reported a modest profit increase Tuesday as it boosts investment in US stores amid a unionization campaign. The coffee giant scored a 12 percent jump in comparable sales in North America, while suffering a 23 percent slide in China amid that country’s latest Covid-19 outbreak. Interim Chief Executive Howard Schultz said the chain was ramping up investments in “high-returning”…