Spring Improve test coverage of StringUtils

HI 🙋🏻‍♀️

I realized that some methods in StringUtils didn't have test cases, so I added them.

Comment From: snicoll

@zinzoddari thank you for making your first contribution to Spring Framework. I've polished things up in https://github.com/spring-projects/spring-framework/commit/ec9d29ea8f299449472bc9810fed992970fdc7a7.