gh-102967: Don't pass None to seek(), simply truncate from the current position (GH-102968)

(cherry picked from commit 7586115)

Co-authored-by: aaron-fl <43595516+aaron-fl@users.noreply.github.com>

Author: aaron-fl

Lib/doctest.py

@@ -291,7 +291,8 @@ def getvalue(self):
         return result
 
     def truncate(self, size=None):
-        self.seek(size)
+        if size is not None:
+            self.seek(size)
         StringIO.truncate(self)
 
 # Worst-case linear-time ellipsis matching.

Misc/ACKS

@@ -419,6 +419,7 @@ Lars Damerow
 Evan Dandrea
 Eric Daniel
 Scott David Daniels
+Aaron Davidson
 Derzsi Dániel
 Lawrence D'Anna
 Ben Darnell

Misc/NEWS.d/next/Library/2023-03-23-19-47-05.gh-issue-102967.gfIYkN.rst

@@ -0,0 +1,2 @@
+A bug in :func:`!doctest._SpoofOut.truncate` was causing None to be passed to :func:`!StringIO.seek` when no size was given.
+A simple fix skips the seek call when no size is given so the buffer can be truncated from the current position.