gh-151857: Fix IndexError in the email header parser on empty input

[tonghuaroot]

Fixes two instances of a single bug class in the email header parser: an empty token-list/string is indexed and escapes as a bare IndexError rather than being handled as a parse defect.

1. get_parameter — a parameter name ending in the extended-parameter marker * with no value leaves value empty before value[0]. Guard: if not value or value[0] != '='.
2. DisplayName.display_name — a display name consisting only of a comment empties res after res.pop(0), then res[-1] raised. Guard: early-return res.value ('') when res is empty after the pop, mirroring the pre-existing len(res) == 0 guard.

Both now degrade gracefully (a parse defect / an empty display name).

Surveyed both instances and closed the class: a focused fuzz of malformed address and parameter headers — 23.5k structured combinations plus 100k randomized message_from_string parses under email.policy.default — shows no other non-HeaderParseError exception escaping the parser. The empty-string IndexError still produced by directly calling the low-level get_* token parsers is their documented non-empty precondition and is unreachable from public header parsing (every caller checks non-empty first).

Tests: regression tests for both instances at the parser level (test__header_value_parser.py) and via the public header API (test_headerregistry.py). The previous name*; charset=utf-8 case did not actually exercise the regression — it raises HeaderParseError on both patched and unpatched code — and has been replaced with cases that IndexError unpatched (name*0*, x=1; name*).

• Issue: email: uncaught IndexError in the header parser on empty-input edge cases #151857