utils/server.lua: don't assume player association

Until now, it was assumed that entity "character" was player associated.
While this is true in 99% of cases, mods that allows NPCs are not
associated with a player and thus when they kill, error happens.

This patch adds additional check.

utils/server.lua

@@ -730,7 +730,7 @@ Event.add(
             message[#message + 1] = ' was killed by '
 
             local name = cause.name
-            if name == 'character' then
+            if name == 'character' and cause.player then
                 name = cause.player.name
             end