Use lazy fields accessor for empty header operations

[OskarEichler]

Summary

Use the existing fields accessor when copying a header or collecting its errors. A newly constructed empty header has no @fields yet, so dup/clone/errors currently call methods on nil.

Reproduction

Run in a fresh process against the gem:

require 'mail'
header = Mail::Header.new
p header.errors # => []
p header.dup.encoded # => empty string
p header.clone.encoded # => empty string

Verification

• Existing bundle exec rake spec: 1,837 examples, zero failures, four existing pending, both before and after this isolated patch, on Ruby 4.0.6 via rbenv.
• External focused checks: 51 checks; 14 baseline failing expectations → zero.
• Nil/empty/populated headers; dup and clone; frozen originals; encoded contents; independent fields; empty errors. Existing populated-header copying behavior and clone frozen state retained.
• Targeted syntax and git diff --check pass. Supplementary Lint output retains the same 275 existing findings by file/cop/message.

Limitations and contribution policy

CONTRIBUTING.md and relevant existing specs were reviewed; a changelog entry is included. The requesting project's explicit no-new-tests policy prevents adding or modifying repository specs, so this PR supplies an inline reproduction and runs the existing suite plus external checks. This differs from upstream's request for a regression spec and may need maintainer follow-up. No claim of coverage completeness, other Ruby engines/versions, production throughput, or live delivery. Verification uses an external bundle with the consumer's patched network/mini_mime dependencies; benchmark is supplied externally for Ruby 4 (existing #1655/#1666), without changing the gem dependencies or CI.

Breaking changes

No intended breaking changes. Empty-header operations now succeed; the general shallow-freeze contract is unchanged.