ErrorMvcAutoConfiguration creates a BeanNameViewResolver bean in case
the user has used @EnabledWebMvc and disabled WebMvcAutoConfiguration.
If the user hasn’t used @EnabledWebMvc, WebMvcAutoConfiguration will
creates its BeanNameViewResolver and override the one that’s already
been defined by ErrorMvcAutoConfiguration.

This commit makes WebMvcAutoConfiguration’s BeanNameViewResolver
definition conditional on there being no existing BeanNameViewResolver
bean definition.

Closes gh-5354