Delegate getExecutionContextSerializer() to DefaultBatchConfiguration if not provided

[izeye]

Like other delegations in SpringBootBatchConfiguration, it would be better to delegate getExecutionContextSerializer() to DefaultBatchConfiguration if a custom one is not provided rather than create one based on the current implementation of DefaultBatchConfiguration.getExecutionContextSerializer().

See gh-38328

[scottfrederick]

Good catch @izeye, thanks!

[benelog]

With a similar intention, I revised it at https://github.com/spring-projects/spring-boot/pull/38631/files

this.serializer = executionContextSerializer.getIfAvailable(super::getExecutionContextSerializer);