Refine null-safety with NullAway build-time checks

[sdeleuze]

As discussed with the JSpecify working group, the goal of this issue is to introduce null-safety checks at build-time using NullAway and the related Error Prone Gradle plugin and refine Spring null-safety annotations accordingly.

Unlike #28797, the purpose of this issue is to continue to leverage JSR 305 annotations. That said, such formal build-time checks will likely ease a potential transition to a different null-safety mechanism (JSpecify or at Java platform level) in the future.

See this draft commit that enables those null-checks on spring-core and generates 41 errors that will likely to lead null-safety refinements.