Improve consistency in StandardMultipartFile implementation

[nuheajiohc]

This PR refactors the StandardMultipartFile class to enhance consistency

Changes include:

1. Removed the transferTo(Path dest) method from StandardMultipartFile, as it is already provided as a default method in the MultipartFile interface.
2. Updated the transferTo(File dest) method to use getInputStream() for consistency with the MultipartFile interface's default transferTo(Path dest) implementation.
3. Modified the isEmpty and getBytes methods to also utilize internal methods (getSize() and getInputStream() respectively) for better code consistency.

[snicoll]

Thanks for the PR but I think the code is fine as it is. The internal methods call what is expected, irrespective of the fact they implement a public contract as well. For instance, we may return a different InputStream than the one we'd be using internally to copy to a byte array.

There are other places in the codebase where we do this. If we decided to change that, then we should be doing that consistently, not in an isolated class.