feat: casting empty numberstring to null

[sodoojin]

Description

In the MVC web service, when receiving user input as object from the controller through form submit, each fields should be casted from string to suitable type.
and class-transformer returns 0 when converting empty string to number.

so, IsNotEmpty of class-validator doesn't work for number field because the number field user not input will be initialized to 0.

If class-transformer returns null when converting empty characters to numbers, then IsNotEmpty can be used for number fields as well.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

[diffy0712]

I do not think this should be added. This could cause other issues for existing users of this library.
I do believe that this problem should be handled via a custom Transformer in your application code.
Edit: In js Number('') will return 0, which is expected.