The builder method required a non-null contextPath while the underlying
field, MutatedServerHttpRequest constructor, and RequestPath.parse all
accept null and treat it the same as an empty string. Relax the method
parameter to @Nullable so callers can clear the context path directly.
Closes gh-37099
Signed-off-by: Garvit Joshi <garvitjoshi9@gmail.com>
