simplify application file encoding detection，return directly when the condition is not met

Comment From: snicoll

Thanks for the PR but I think we should try to restrict the use of return which the original code does. There is a case to be made that we check if the property is set and then we get its value which can be simplified. I've done that in https://github.com/spring-projects/spring-boot/commit/fb2f6bb1afb69ec9623e4776818114609d00d574. Thanks again for the PR in any case.

Comment From: refeccd

Well, aside from the use of return, why does the encoding attribute need to be judged again? springboot Simplify application file encoding detection The non-null check has been done in the equalsIgnoreCase method, why do we need to repeat？

Comment From: snicoll

Because we don't want to run the check at all if the system property is not set.