Restore no-op default. (#3879)

As it stands, if there's only one non-disabled radio button, using up/down to navigate will de-select it (because the default value in 'min' is None) when navigation should do nothing (the default should be 'selected').

src/textual/widgets/_radio_set.py

@@ -299,7 +299,7 @@ def distance(index: int) -> int:
                 """
                 return direction * (index - selected) % len(self.children)
 
-            self._selected = min(candidate_indices, key=distance, default=None)
+            self._selected = min(candidate_indices, key=distance, default=selected)
 
     def action_toggle(self) -> None:
         """Toggle the state of the currently-selected button."""