Prior to this commit, the `HttpHandlerAutoConfiguration` would rely on
the `WebFluxProperties` bean being present - this is most of the time
true when the appplication is using the WebFlux auto-configuration.
If the application is overriding the WebFlux auto-configuration and
providing its own setup, the properties bean is not present and we
should skip its usage.
Fixes gh-20891
0611b6d3eb