Add missing docstring

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/_select.py

@@ -152,6 +152,7 @@ class _EmptyResult:
     """
 
     def __repr__(self) -> str:
+        """Return a string with the internal representation of this instance."""
         return "<empty>"