owpaintdata: Simplify code

Author: lanzagar

Orange/widgets/data/owpaintdata.py

@@ -797,7 +797,7 @@ def __init__(self):
         self.class_model.rowsInserted.connect(self._class_count_changed)
         self.class_model.rowsRemoved.connect(self._class_count_changed)
 
-        if self.data is None or not len(self.data):
+        if not self.data:
             self.data = []
             self.__buffer = np.zeros((0, 3))
         elif isinstance(self.data, np.ndarray):