Refine JSpecify annotations

[mhalbritter]

Continuation of #46926.

High

• Add NullAway checking to test code

Medium

• Review org.springframework.boot.test.json.JsonContentAssert#compare(java.lang.CharSequence, org.skyscreamer.jsonassert.JSONCompareMode) when passing null as first parameter

Low

• Remove varargs checkstyle suppressions (needs release of https://github.com/spring-io/spring-javaformat)
• Depending on SingletonSupplier is not singleton if null is returned on first supplier call spring-framework#35369: Refactor SingletonSupplier usages to only use them with non-nullable suppliers

To decide

• Do we want to null-check buildSrc?
  • We don't bother right now
• Do we want to null-check spring-boot-configuration-metadata?
  • We don't bother right now
• Do we want to null-check spring-boot-configuration-metadata-changelog-generator?
  • We don't bother right now
• Do we want to null-check / add annotations to spring-boot-docs?