fix(box): handle edge cases for "ignore:" option

[dailyrandomphoto]

What does it do?

Continue of #3908 (#3904 (comment))

Related: #3904 , #3538

The first commit fails on new edge cases.

ignore: 
  -   # <= null

ignore: 
  -   # <= null
  - /some/path/

or

ignore: 
  - '' # <= empty string

The second commit will resolve this issue.

How to test

git clone -b fix-ignore-null https://github.com/dailyrandomphoto/hexo.git
cd hexo
npm install
npm test

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.136% when pulling ce201f9 on dailyrandomphoto:fix-ignore-null into 5d00e02 on hexojs:master.

[coveralls]

Coverage remained the same at 97.136% when pulling 3c93f8f on dailyrandomphoto:fix-ignore-null into 5d00e02 on hexojs:master.

[curbengh]

Sorry, I think this PR is being too lenient on invalid config.