fix: set default highlight arg to blank buffer

when not refreshing the highlight layer, it can corrupt over time,
observed in the `epd2in13b_V3` model. So we refresh it with the blank
buffer even when there are no highlight pixels

tinyticker/waveshare_lib/_base.py

@@ -154,7 +154,7 @@ def clear(self) -> None:
 
     def show(self, image: Image.Image) -> None:
         threshold = 20
-        highlight_buffer = None
+        highlight_buffer = self._blank
         if image.mode == "RGB":
             highlight_mask = np.array(image).std(axis=-1) >= threshold
             if highlight_mask.any():