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.cpp

@@ -28,6 +28,10 @@ void SleepActivity::onEnter() {
   Activity::onEnter();
   renderPopup("Entering Sleep...");
 
+  if (SETTINGS.sleepScreen == CrossPointSettings::SLEEP_SCREEN_MODE::BLANK) {
+    return renderBlankSleepScreen();
+  }
+
   if (SETTINGS.sleepScreen == CrossPointSettings::SLEEP_SCREEN_MODE::CUSTOM) {
     return renderCustomSleepScreen();
   }
@@ -234,3 +238,8 @@ void SleepActivity::renderCoverSleepScreen() const {
 
   renderDefaultSleepScreen();
 }
+
+void SleepActivity::renderBlankSleepScreen() const {
+  renderer.clearScreen();
+  renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+}