Return a value when stacktrace is empty.

src/Exception.hpp

@@ -37,6 +37,9 @@ namespace ArbUt {
         [[nodiscard]] std::string GetStacktrace([[maybe_unused]] size_t depth = 6,
                                                 [[maybe_unused]] bool include_addr = true) const {
 #if !WINDOWS
+            if (_stack.size() == 0){
+                return "No stack trace could be retrieved.";
+            }
             backward::TraceResolver tr;
             tr.load_stacktrace(_stack);
             std::stringstream ss;