After two years of horror stories about the alleged mass graves of indigenous children at residential schools across Canada, a series of recent excavations at suspected sites has turned up no human remains. Some academics and politicians say it’s further evidence that the stories are unproven. Minegoziibe Anishinabe, a group of indigenous people also known as Pine Creek First Nation, excavated 14 sites in the basement of Our Lady of Seven Sorrows Catholic Church near the Pine Creek Residential School in Manitoba during four weeks this summer. The so-called “anomalies” were first detected using ground-penetrating radar but on Aug. 18,...