perf(meta_generator): avoid unnecessary check

[SukkaW]

What does it do?

This PR is mean to supersede #4070 and #4078.

With cached value NEED_INJECT, all the checks will be only executed once.

How to test

git clone -b meta_generator_optimize https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[SukkaW]

According to the benchmark, the generation duration dropped by 3% after this PR.

[yoshinorin]

LGTM :)