Fixes https://github.com/spring-projects/spring-integration/issues/8750 * annotate `IntegrationFlowRegistration.getRegistrationById(String flowId)` with `@org.springframework.lang.Nullable` * annotate `StandardIntegrationFlowContext.getRegistrationById(String flowId)` with `@org.springframework.lang.Nullable` **Cherry-pick to `6.1.x` & `6.0.x`**