Client: Make trade route drawing robust against unknown partner
Related to #42968
Currently player is expected to always know any trade partners of his/her own cities, but 1) it won't hurt for the client to be robust against the opposite case, and 2) that might not be true sometime in the future.
This ticket is about changing draw_trade_routes_for_city() not to crash when the trade partner from trade_partners_iterate() is NULL.
Even when #42968 gets fixed for the future servers, this ticket helps clients to work with older servers.
S2_6 is not affected, as there the iterator macro used contains NULL check.
Targeting to beta3, even with less than one week to go:
- Avoids a crash, one that presumably occurs quite regularly in longer games
- Is trivial (unlike the fix to actual #42968)
- Crash is a regression since 2.6 -> to anyone switching branches