Previously, reflection was used to set the OBJENESIS field of
DummyInvocationUtils with an Objenesis instance that does not use
caching. This has stopped working as the field is now declared final.

This commit updates the approach take by HateoasObjenesisCacheDisabler
to disable Objenesis's cache. Rather than changing the value of the
OBJENESIS field on DummyInvocationUtils, the cache field on the
ObjenesisStd instance is set to null instead. This has the desired
effect of disabling Objenesis's caching.

See gh-3784
Closes gh-8335