Remove asserts the types already cover

The nil check burn down added guards which turned out redundant once
the surrounding annotations landed. `_has_handlers` is set to true when
the first handler registers, so it is a boolean rather than the literal
false it was inferred as.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_scenario/module/control/discord_alerts.lua

@@ -32,7 +32,7 @@ end
 
 --- Get the player name from an event
 --- @param event { player_index: uint, by_player_name: string? }
---- @return string, string
+--- @return string, string?
 local function get_player_name(event)
     local player = game.players[event.player_index]
     return player.name, event.by_player_name