[blihio]

Fixes #3513

PaginatedResult.getRequestForNextResult() and StorageListResult.getNextToken() return null for terminal pages. Add @Nullable to both getters and document that condition so Kotlin callers see nullable types and handle the end of pagination explicitly.

This preserves runtime pagination behavior and JVM method declarations. Recompiled Kotlin callers that assume non-null return values must handle null explicitly.

Validation on Amplify Android 2.43.0:

• Actual Java API classes and Kotlin 2.2.21 consumers: original getters allow non-null assignments that throw on terminal data; annotated getters reject those assignments at compilation. Safe-call consumers compile and run with both versions.
• :core:testDebugUnitTest: 163 tests pass, zero failures, errors or skips.
• :core:checkstyle and :core:lintDebug pass.
• javap -public declarations are unchanged.