Add blank sleep screen option (#242)

## Summary

Very small change to add a blank ("None") sleep screen option, for those
who prefer a clean aesthetic.

Tested on X4 device.

src/activities/boot_sleep/SleepActivity.h

@@ -15,4 +15,5 @@ class SleepActivity final : public Activity {
   void renderCustomSleepScreen() const;
   void renderCoverSleepScreen() const;
   void renderBitmapSleepScreen(const Bitmap& bitmap) const;
+  void renderBlankSleepScreen() const;
 };