The Lestrange bloodline would continue through another branch.
The Lestrange bloodline could continue on through another male member of the bloodline though Leta’s father Corvus Sr. has no living descendants. It doesn’t have to have continued through him to have continued past his generation. It can continue through another branch. On the Lestrange family tree, there are other branches shown reaching nearly as far upwards as Corvus Sr.’s, the one ending with baby Corvus and Leta, meaning those branches likely have members who’d still be alive at a similar time period as Corvus Sr. was and they’d still be able to continue the bloodline.
Corvus Sr.’s is only one branch on the larger Lestrange family tree. The branches that have Fulcran Lestrange and Cyrille Lestrange seem to have a comparable number of generations of Lestranges as Corvus Sr.’s branch, so it has continued to Corvus Sr.’s generation through other branches as well. One of those branches will continue it further, since a new Lestrange will have be born to one of them sometime in the 1920s or 1930s. The baby may not have appeared on the tree yet, but the bloodline continues past Corvus Sr.’s generation, since a Lestrange baby was either already born or will be. There’s a male Lestrange who went to Hogwarts when Tom Riddle did, meaning he’d be born somewhere around 1926 since he’d have to be born, at the most, seven years apart from Tom for his attendance at Hogwarts to overlap with Tom - the bloodline would have continued with his parents then possibly through him, as being male, he could pass on the Lestrange name.
“Good gracious, is it that time already?’ said Slughorn. ‘You’d better get going, boys, or we’ll all be in trouble. Lestrange, I want your essay by tomorrow or it’s detention.”
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
One of those other branches would have the parents of the Lestrange who went to Hogwarts with Tom Riddle - he might not have been born yet, so he may not be on the tree. It’s not clear how he’s related to Corvus Lestrange Sr. but he was born to parents that were Lestranges, since he’s born a Lestrange. The line was clearly able to continue through another branch than Corvus Sr.’s, and if that Lestrange wasn’t born when the tree is seen, wouldn’t be shown yet.