Fix comments that are different from the behavior

[heowc]

I'm using StringUtils.equals and found that the documentation and behavior are slightly different. I'll update the documentation and add tests to match the behavior.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)

Checklist

• [x] I have read the CONTRIBUTING document
• [x] Local run of mvn install succeeds
• [x] My code follows the code style of this project
• [x] My change requires a change to the Javadoc documentation
• [x] I have updated the Javadoc documentation accordingly
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed
• [ ] I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• [ ] My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• [x] I confirm that this pull request can be released under the Apache 2 license

[heowc]

It's been a while since I submitted a PR. If there's anything wrong with solving the issue, please let me know 🤔

[iparnelly]

There are 2 comments that still state that 2 nulls are equal.

     * <p>{@code null}s are handled without exceptions. Two {@code null}
     * references are considered to be equal. The comparison is case sensitive.</p>

and

     * @return {@code true} if the Strings are equal (case-sensitive), or both {@code null}