test: allow empty attribute

[curbengh]

What does it do?

asset_img() and link() utilize hexo-util. hexojs/hexo-util#36 introduced a change, where empty attributes (attribute without any value) are retained, instead of being removed.

This PR modifies the test cases to adapt to new behavior of hexo-util@1.0.0.

How to test

git clone -b unit-test https://github.com/curbengh/hexo.git
cd hexo

package.json:
- "hexo-util": "^0.6.3",
+ "hexo-util": "1.0.0-rc1",

npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.
  • Will only pass after hexo starts using hexo-util@1.0.0.

[coveralls]

Coverage remained the same at 97.152% when pulling 6b395fc on curbengh:unit-test into 9b87047 on hexojs:master.

[coveralls]

Coverage remained the same at 97.152% when pulling 6b395fc on curbengh:unit-test into 9b87047 on hexojs:master.

[curbengh]

Merged into #3646