Fix phpdoc @param tags with the NULL type

[wulff]

Brief Summary of Changes

Parameters documented by @param null make it difficult for IDEs and static analysis tools to reason about the code. This PR tries to fix the most obvious occurences of the pattern.

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [ ] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests
• [x] Documentation

Are tests included?

• [ ] Yes
• [x] No This PR only contains changes to documentation.

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all of the tests pass.

[const-cloudinary]

@wulff , thank you for contribution!