Make author image optional

[matheusgeres]

PR Checklist

• The commit message follows our guidelines.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Currently in docs there is a behavior that includes doesn't show author image when is not present. But even not present im YAML an image is rendering.

Issue resolved: N/A

What is the new behavior?

Author image is showing only there is a author image on YAML.

• Screenshots with this changes:
  

• Link to demo site with this changes:
  https://www.geresdev.com.br/

How to use?

In Hexo next.yml don't include avatar.url

Does this PR introduce a breaking change?

• Yes.
• No.

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