maintain type stability on return value

Co-authored-by: Thomas A Caswell <[email protected]>

Author: raphaelquast

lib/matplotlib/backend_bases.py

@@ -3014,7 +3014,7 @@ def _capture_events(ax):
         axes = list(filter(_ax_filter, self.canvas.figure.get_axes()))
 
         if len(axes) == 0:
-            return
+            return []
 
         if self._nav_stack() is None:
             self.push_current()   # Set the home button to this view.