fix(#4780): empty tag name correction

[SukkaW]

What does it do?

Fix #4780 (comment)

cc @stevenjoezhang

How to test

git clone -b fix-post-render-escape-empty-tag https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Pull request tasks

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

[coveralls]

Coverage decreased (-0.03%) to 98.136% when pulling b32252b on SukkaW:fix-post-render-escape-empty-tag into b3bd7d4 on hexojs:master.