chore: drop unnecessary | None on LLMError and Mail.send

[BrianWang1990]

Summary

Refs #35557.

A couple of small spots where the | None annotation was carrying no
real information and the body never distinguished between None and a
plain empty string. Tighten them to str with a "" default so the
types match actual usage.

This is a follow-on cleanup in the same spirit as #36135, but for
different files.

Changes

• api/core/errors/error.py: LLMError.description and
  LLMError.__init__(description=...) — str | None = None → str = "".
  No subclass or caller relies on description is None; the field is only
  set/read as a string.
• api/extensions/ext_mail.py: Mail.send(from_=...) —
  str | None = None → str = "". The body uses if not from_: then falls
  back to self._default_send_from, so an empty string and None are
  already equivalent. The only non-test caller
  (tasks/mail_human_input_delivery_task.py) does not pass from_.

Notes

• I deliberately kept the diff tiny. There are more candidates suggested
  in the issue thread, but several of them have call sites that pass
  None explicitly or rely on is None semantics, so they need more
  thought than a mechanical rewrite.
• No behavior change. No new tests required: existing email-send tests
  still cover the default-from_ path, and LLMError.description has no
  test that checks the None case.

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	43.70%	+43.70%
Strict coverage	0.00%	43.22%	+43.22%
Typed symbols	0	22,024	+22,024
Untyped symbols	0	28,690	+28,690
Modules	0	2551	+2,551