This commit revises the fix submitted in 959e6d1745 by ensuring that
empty file input is converted to a MockPart with the supplied name, an
empty filename, "application/octet-stream" as the content type, and
empty content.
This aligns with the behavior of Servlet containers, as tested with the
interaction between Firefox and a standard Servlet running in a Jetty
Servlet container.
Closes gh-26799