Since https://jira.spring.io/browse/SPR-16624, the contract for
`ContentNegotiationStrategy` has been refined and should never return an
empty list if it's got no preference for a media type, but it should
rather respond with a `"*/*"` instead.

This commit fixes the `OptionalPathExtensionContentNegotiationStrategy`
defined in the Spring MVC auto-configuration to have that behavior.

Fixes gh-12900