Is there an existing issue for this?

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Description of the bug

Header and footer are always rendered in changelog. Removing the fields in cliff.toml will render the fallback default values of the config.

Steps To Reproduce

1. Init git-cliff
2. Remove header or footer fields in the config
3. Render a changelog

Expected behavior

Header and footer should be removed when the fields do not exist.

Screenshots / Logs

No response

Software information

• Operating system: AOSC OS 12.2.0
• Rust version: 1.87.0
• Project version: 2.9.1

Additional context

No response