fix(save_database): should not clear database when use 'hexo new', 'hexo --help', etc.

[dailyrandomphoto]

What does it do?

After run hexo commands such as hexo --version, hexo new,
the content of `db.json' file was cleared.
It is inconvenient for a site with a lot of posts.

This PR fixes this issue by saving the database only when the database was loaded.

How to test

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

git clone https://github.com/hexojs/site.git
cd site
vi package.json

Change the version of hexo.

- "hexo": "hexojs/hexo"
+ "hexo": "dailyrandomphoto/hexo#fix-save-database"

Install and generate site.

npm install
npm run build

Check out the db.json file.

hexo --version
# or 
hexo new page "somepage"

Check out the db.json file.
The content of db.json should not be cleared.

Screenshots

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.269% when pulling bc9748f on dailyrandomphoto:fix-save-database into d8ec214 on hexojs:master.