Spring MockHttpServletRequestBuilder should merge URI from the parent if necessary

As of #32913, Most of MockHttpServletRequestBuilder has moved to an abstract class that can only be built with an HttpMethod whereas previously both an HttpMethod and an URI were mandatory.

Moving URI as nullable means it can be inherited from a parent builder if it is not set yet. This isn't taken into account and can be a problem if you need to create a separate builder instance from one whose URI has been populated already.