[ConstraintSystem] In selectDisjunction, bail early if we find no disjunctions.

Author: rudkx

lib/Sema/CSSolver.cpp

@@ -1938,6 +1938,9 @@ Constraint *ConstraintSystem::selectDisjunction() {
   SmallVector<Constraint *, 4> disjunctions;
 
   collectDisjunctions(disjunctions);
+  if (disjunctions.empty())
+    return nullptr;
+
   if (auto *disjunction = selectBestBindingDisjunction(*this, disjunctions))
     return disjunction;