SpringBoot Assert that sources passed to the Binder during construction do not contain null elements

See https://github.com/spring-cloud/spring-cloud-commons/issues/1062. The error was hard to find in part because we don't assert that sources have no null elements.

Comment From: snicoll

Looking at the related issue, that sounds more like a bug to me.

Comment From: philwebb

Closing in favor of PR #30878. Thanks @terminux!