dont create a background bitmap if background_color is None

Author: FoamyGuy

adafruit_display_text/label.py

@@ -292,7 +292,8 @@ def _update_text(
         self._text = new_text
         self._boundingbox = (left, top, left + right, bottom - top)
 
-        self._update_background_color(self._background_color)
+        if self.background_color:
+            self._update_background_color(self._background_color)
 
     @property
     def bounding_box(self):