chore(Connection): Fix warning due to non-void function not returning

In release builds, the assert is omitted and the function shows undefined
behavior due to not returning.
We now return nullptr to avoid UB; in general we expect developers to build
in non-release mode, and to encounter the assert when code wrongly dispatches
to this function.

Author: muffgaga

nestkernel/connection.h

@@ -402,7 +402,8 @@ template < typename targetidentifierT >
 SecondaryEvent*
 Connection< targetidentifierT >::get_secondary_event()
 {
-  assert( false );
+  assert( false && "non-primary connections have to provide get_secondary_event()");
+  return nullptr;
 }
 
 } // namespace nest