wrap_own_share: add overload for `nullptr_t` to reset shared pointer

dune/solvers/common/wrapownshare.hh

@@ -34,6 +34,12 @@ namespace Dune {
         return std::make_shared<S>(std::move(t));
       }
 
+      template<class T>
+      std::shared_ptr<T> wrap_own_share(std::nullptr_t)
+      {
+        return {};
+      }
+
       //! Share ownership of shared_ptr
       template<class T, class S>
       std::shared_ptr<T> wrap_own_share(std::shared_ptr<S> t)