`html.parser`: `check_for_whole_start_tag` crashes with `AssertionError` on empty string

[T90REAL]

Bug report

Bug description:

The html.parser.HTMLParser method check_for_whole_start_tag crashes with an AssertionError when its rawdata is an empty string.

from html.parser import HTMLParser

parser = HTMLParser()

parser.feed('')

result = parser.check_for_whole_start_tag(0)

Traceback (most recent call last):
  File "/data/src/test.py", line 11, in <module>
    result = parser.check_for_whole_start_tag(0)
  File "/home/hdd/miniconda3/lib/python3.13/html/parser.py", line 394, in check_for_whole_start_tag
    raise AssertionError("we should not get here!")
AssertionError: we should not get here!

I think the internal code should gracefully handle this None match by returning -1 instead if an AssertionError?

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[T90REAL]

Not only empty string, but also some incomplete tag such as '<ª' would trigger this assertion.

[efimov-mikhail]

cc @serhiy-storchaka because of 135930

[serhiy-storchaka]

This works as expected. check_for_whole_start_tag() should only be called if the next characters are < followed by an ASCII letter. Assertions just check an internal consistency of the code.