ResearchTree: add a missing early return.

src/ResearchTree/ResearchTree.cpp

@@ -381,6 +381,7 @@ void ResearchTree::generateArchTree() {
 void ResearchTree::readEngineUnlocks(Containers::ArrayView<Int> engines, Containers::ArrayView<Int> gears) {
     if(engines == nullptr || engines.size() == 0) {
         Utility::Error{} << "Engines can't be empty";
+        return;
     }
 
     for(Int& engine : engines) {