Prior to this commit, MockHttpServletRequestBuilder always supplied an
array of cookies to the MockHttpServletRequest that it built, even if
the array was empty.
However, this violates the contract of HttpServletRequest. According to
the Servlet API, the getCookies() method "returns null if no cookies
were sent."
This commit ensures that MockHttpServletRequestBuilder no longer
configures an empty array of cookies in the mock request that it builds.
Issue: SPR-13314
93c07e76bc