Initialize distro_series as optional

src/source_package_publishing_history.cpp

@@ -37,7 +37,7 @@ source_package_publishing_history::source_package_publishing_history()
 
           if (ds) {
               ds->set_lp(lp);
-              _distro_series = ds;
+              _distro_series = std::make_optional(ds.value());
               return ds;
           }