fix: clear input value on clearFiles from imperative handle

[Steven7926]

Summary

A simple change that clears the input elements value in a file input component when the files state is cleared through the imperative handle.

First PR so any feedback would be greatly appreciated. Thanks!

Closes #3089

How To Test

1. Use the FileInput component with ref and custom handlers that has a "clear files" button. Story Book example: https://trussworks.github.io/react-uswds/?path=/story/components-file-input--with-ref-and-custom-handlers
2. Add a file to the FileInput component.
3. Clear the file using the clear files button.
4. Ensure the file clears.
5. Add a new file to the FileInput
6. Ensure the event is fired and the file i added properly to the component.

[brandonlenz]

Per #3089 (comment) I was going to leave slightly more suggestions, but after a bit of thought, nothing I suggest here will actually relieve the code smells we have. The only suggestion I'll make, to help us make clear that this example code is managing its own state in an attempt to mirror what is going on internally in the FileInput component, is to pull this null into a named constant, e.g.:

Suggested change

	setFiles(null)
	setFiles(INITIAL_FILES)

[brandonlenz]

Suggested change

	const [files, setFiles] = useState<FileList | null>(null)
	const INITIAL_FILES = null
	const [files, setFiles] = useState<FileList | null>(INITIAL_FILES)