perf(open_graph): only require cheerio when content contains '<img'

[SukkaW]

What does it do?

only require cheerio when content contains '<img'

How to test

git clone -b BRANCH https://github.com/USER/hexo.git
cd hexo
npm install
npm test

Screenshots

#3663 (comment)

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.0008%) to 97.155% when pulling d6dce10 on SukkaW:lazy-cheerio into 3176fe0 on hexojs:master.

[curbengh]

Is if (!images.length && content && content.includes('<img')) safe? I'm wondering if content can be null. Let me try with an empty file.

---

No issue/error with an empty file. Even with an empty file, it still contains basic stuff defined in a theme. So, I assume content will never be empty.

[SukkaW]

Maybe some syntax error might cause content to be null, I'm not sure.

[curbengh]

My preference is on #3680

[SukkaW]

Superseded by #3680