Kernel: Don't attempt to yield execution in SleepThread(0) if there are no available threads to run.

With this we avoid an useless temporary deschedule of the current thread.

src/core/hle/kernel/thread.cpp

@@ -556,6 +556,10 @@ SharedPtr<Thread> SetupMainThread(u32 entry_point, s32 priority) {
     return thread;
 }
 
+bool HaveReadyThreads() {
+    return ready_queue.get_first() != nullptr;
+}
+
 void Reschedule() {
     PriorityBoostStarvedThreads();