LibCore: Fix timer expiration processing

The previous code did not account for the case when there are timers present,
but none are enabled, and used a zero polling timeout.

Fixes https://github.com/SerenityOS/serenity/issues/2222

Libraries/LibCore/EventLoop.h

@@ -77,7 +77,7 @@ public:
 
 private:
     void wait_for_event(WaitMode);
-    void get_next_timer_expiration(timeval&);
+    Optional<struct timeval> get_next_timer_expiration();
 
     struct QueuedEvent {
         AK_MAKE_NONCOPYABLE(QueuedEvent);