OncePerRequestFilter.isAsyncDispatch may return a NPE

[slyoldfox]

This is a follow-up ticket for as request in #26282 where OncePerRequestFilter.isAsyncDispatch might NPE.

In our case we were using a silly mockito mock in a test which was testing the MultiPartFilter integration

HttpServletRequest request = mock( HttpServletRequest.class );
HttpServletResponse response = mock( HttpServletResponse.class );

multipartFilter.doFilter( request, response, mock( FilterChain.class ) );

[rstoyanchev]

We can change it in this case because if you wanted an ASYNC dispatch, you'd have to set it up. Per contract however getDispatcherType() isn't supposed to return null. There are other places where we test for REQUEST and defaulting to "false" there wouldn't be a good idea.