Fix panic when sending an empty file field in a form

[Supet1337]

Fix panic on empty file field in RawBody

Problem

When an empty file field is submitted in a form (multipart/form-data), form.Value contains an empty string (""), but form.File is missing. This leads to a panic with the error: unsupported param type huma.FormFile

Solution

Now, before accessing form.Value[p.Name], we check if the value is an empty string and handle it correctly. If the parameter is required and not a valid file, a proper validation error is returned instead of causing a panic.