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/gui/quick_actions.lua

@@ -43,7 +43,7 @@ local function new_quick_action(command, on_click)
         :style{
             width = 160,
         }
-        :on_click(on_click or function(def, player, element, event)
+        :on_click(on_click or function(_def, player, _element, _event)
             command(player)
         end)