[Bug] jest mock returns null when falsy values are stored

[shrouxm]

Describe the bug
When using the jest mock, if you store falsy values and then retrieve them from the store you will get null back.

For example, in a test file:

mmkv.setBool('key', false);
mmkv.getBool('key'); // returns null

mmkv.setString('key', '');
mmkv.getString(''); // returns null

Expected behavior
Should return the falsy value instead of null

Additional context
I think this PR intended to fix the issue, but it updates the .ts file without updating the .js file in the dist folder so users are still running code with the bug.

I'm not sure how to generate a new dist folder so hoping that will be an easy step for maintainers!