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.h

@@ -218,6 +218,11 @@ private:
  */
 SharedPtr<Thread> SetupMainThread(u32 entry_point, s32 priority);
 
+/**
+ * Returns whether there are any threads that are ready to run.
+ */
+bool HaveReadyThreads();
+
 /**
  * Reschedules to the next available thread (call after current thread is suspended)
  */