Add @conditionalOnBean to GraphQLSpringWebfluxAutoConfiguration

[endj]

Unless there is a good reason not to, could

@ConditionalOnBean({GraphQLSchema.class})

be added to the GraphQLSpringWebFluxAutoConfiguration. Currently, things will break if you have the library on the classpath but only instantiating the bean conditionally. It would be a lot nice if you did not have to use exclude the autoconfiguration class.