emscripten: Fix crash on Safari when probing gamepad rumble support

Safari's older Gamepad API exposes `vibrationActuator` with `playEffect`
and `reset` but no `effects` enumeration array. The probe added in
651136ac7 dereferences `vibrationActuator['effects']['includes']`
unconditionally, throwing `TypeError: undefined is not an object` on
every Safari client that opens a connected gamepad. Add the missing
`['effects']` null check so the probe returns false on Safari instead
of aborting.

(cherry picked from commit db7ac820f9c95dd75c7b71e6d37231534b6c13c5)

src/joystick/emscripten/SDL_sysjoystick.c

@@ -506,7 +506,7 @@ static bool EMSCRIPTEN_JoystickOpen(SDL_Joystick *joystick, int device_index)
 
     item->rumble_available = MAIN_THREAD_EM_ASM_INT({
         let gamepad = navigator['getGamepads']()[$0];
-        return gamepad && gamepad['vibrationActuator'] && gamepad['vibrationActuator']['effects']['includes']('dual-rumble');
+        return gamepad && gamepad['vibrationActuator'] && gamepad['vibrationActuator']['effects'] && gamepad['vibrationActuator']['effects']['includes']('dual-rumble');
         }, item->index);
 
     if (item->rumble_available) {
@@ -515,7 +515,7 @@ static bool EMSCRIPTEN_JoystickOpen(SDL_Joystick *joystick, int device_index)
 
     item->trigger_rumble_available = MAIN_THREAD_EM_ASM_INT({
         let gamepad = navigator['getGamepads']()[$0];
-        return gamepad && gamepad['vibrationActuator'] && gamepad['vibrationActuator']['effects']['includes']('trigger-rumble');
+        return gamepad && gamepad['vibrationActuator'] && gamepad['vibrationActuator']['effects'] && gamepad['vibrationActuator']['effects']['includes']('trigger-rumble');
         }, item->index);
 
     if (item->trigger_rumble_available) {