Quickfix for a bug that sometimes trade partner is not revealed when route established
Related to #42967, and what was written there:
"The problem I'm debugging is that establishing foreign trade route should reveal the source city to the owner of the destination city. In the failing case owner of the destination city has before seen the tile where the source city is later established. send_city_info_at_tile() behavior causes the city not to get revealed. This then lead to client crash when trade partner was NULL there."
Overall the updates about trade partner are buggy (when trading with someone you who doesn't give you shared vision), and are likely to require lot of fixing. Meanwhile we should at least avoid the crashing cases -> quick fix in this ticket.
Additional client side fix -> #42969