Fix parser

gtsam/discrete/SignatureParser.cpp

@@ -34,7 +34,7 @@ std::optional<Row> static ParseConditional(const std::string& token) {
       row.push_back(std::stod(s));
     }
     // if we ended with a '/' then return false
-    if (token.back() == '/') return false;
+    if (token.back() == '/') return std::nullopt;
   } catch (...) {
     return std::nullopt;
   }