gh-134152: Fix UnboundLocalError in email._header_value_parser _get_ptext_to_endchars...

gh-134152: Fix UnboundLocalError in email._header_value_parser _get_ptext_to_endchars (#134233)

Fix an UnboundLocalError that can occur when parsing certain delimited constructs in headers (domain literals, quoted strings, comments). After the fix the _get_ptext_to_endchars returns an empty string if there is no content after the opening delimiter. The calling code is responsible for handling the lack of the trailing delimiter, which it already does; this edge case was the header ending immediately after the opening delimiter.