Prior to this commit, a recent change applied in gh-29125 changed the
behavior of `MockHttpServletRequest` instances. In case of an empty
request body, the returned `InputStream` would be static and could not
be reused across requests.
This could result in `java.io.IOException: Stream closed` exceptions if
a previous request was read.
This commit ensures that a new instance of an empty stream is returned
for each request instance.
Fixes gh-29901
5fd73f0bae