>>> email.utils.parsedate_to_datetime(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/email/utils.py", line 200, in parsedate_to_datetime
    raise ValueError('Invalid date value or format "%s"' % str(data))
ValueError: Invalid date value or format "None"

First, the date value is None, not "None".

Second, why not just return None? parsedate() can be used in code like:

   parsedata(headers.get('Date'))

None is an expected argument if the header "Date" is absent. parsedate_to_datetime() is not compatible with parsedata() in this case.

It was a regression introduced in issue16714. Before that parsedate_to_datetime(None) returned None.