Polish use of constants in StringUtils

[J-unStiN]

I modified the hardcoded '.' and '\' parts to
EXTENSION_SEPARATOR,
WINDOWS_FOLDER_SEPARATOR_CHAR.

The '\' part can be maintained by adding WINDOWS_FOLDER_SEPARATOR_CHAR
to maintain the overall consistency of the code base.

ps. This is my first Spring Framework PR, so even if there are some shortcomings, I will improve it to a good code through future activities.

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[sbrannen]

Hi @J-unStiN,

Congratulations on submitting your first PR for the Spring Framework! 👍

Although we generally prefer to make such minor changes ourselves, I will accept this since it's your first PR.

Also, please note that we do not add @author tags for such minor changes.

Regards,

Sam

[sbrannen]

This has been merged into main.

Thanks

[J-unStiN]

Thank you very very much.

I will never forget this moment.

I have another question.

I don't see any methods in Spring's StirngUtils that check for empty and null values ​​via isBlank, isEmpty. I wonder if it would be okay to create one and post a PR.