Issue #17901: fix TreeBuilder construction for an explicit element_factory=None

Based on report and patch by Aaron Oakley.
diff --git a/Misc/ACKS b/Misc/ACKS
index b47accf..4eb8ef6 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -887,6 +887,7 @@
 Kevin O'Connor
 Tim O'Malley
 Zooko O'Whielacronx
+Aaron Oakley
 James Oakley
 Jon Oberheide
 Pascal Oberndoerfer