Issue #26560: Avoid potential ValueError in BaseHandler.start_response

Initial patch by Peter Inglesby.

diff --cc Misc/NEWS
index 832d1accd441c3289312c9965eaf6d4be96657cf,bdcfebf856545390cd284de5d091220689be89d9..fe3071ecb5400a630e7fc9bd800726fa6f040209
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -226,11 -90,10 +226,14 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #26560: Avoid potential ValueError in BaseHandler.start_response.
+   Initial patch by Peter Inglesby.
+ 
 +- Issue #26567: Add a new function :c:func:`PyErr_ResourceWarning` function to
 +  pass the destroyed object. Add a *source* attribute to
 +  :class:`warnings.WarningMessage`. Add warnings._showwarnmsg() which uses
 +  tracemalloc to get the traceback where source object was allocated.
 +
  - Issue #26313: ssl.py _load_windows_store_certs fails if windows cert store
    is empty. Patch by Baji.