Fix removing the root node in binary_search_tree.py removes the whole tree (#8752)

* fix issue #8715

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Frank-1998

data_structures/binary_tree/binary_search_tree.py

@@ -40,7 +40,7 @@ def __reassign_nodes(self, node: Node, new_children: Node | None) -> None:
             else:
                 node.parent.left = new_children
         else:
-            self.root = None
+            self.root = new_children
 
     def is_right(self, node: Node) -> bool:
         if node.parent and node.parent.right: