RedisIndexedSessionRepository nullability issues

Describe the bug

When upgrading to spring-session-data-redis 4.1.0 we can no longer call org.springframework.session.data.redis.RedisIndexedSessionRepository#setRedisSessionMapper to set a custom session mapper in Kotlin.
This is because the signature of redisSessionMapper expects the BiFunction to return a non-null MapSession, whereas before this was not the case (the result could be a null session).

Observing org.springframework.session.data.redis.RedisIndexedSessionRepository#getSession we see that the result of the BiFunction is expected to be a nullable value:

	private @Nullable RedisSession getSession(String id, boolean allowExpired) {
		// omitted for brevity
		MapSession loaded = this.redisSessionMapper.apply(id, entries);
		if (loaded == null || (!allowExpired && loaded.isExpired())) {
			return null;
		}
		// omitted for brevity
	}

To Reproduce

The custom mapper below fails to compile in Kotlin because it returns null when loading an invalid session causes an IllegalStateException to be thrown:

    @Bean
    fun redisSessionRepositoryCustomizer() =
        SessionRepositoryCustomizer<RedisIndexedSessionRepository> { redisSessionRepository ->
            val delegate = RedisSessionMapper()
            redisSessionRepository.setRedisSessionMapper { sessionId, map ->
                try {
                    delegate.apply(sessionId, map)
                } catch (_: IllegalStateException) {
                    redisSessionRepository.sessionRedisOperations.delete("${sessionDataRedisProperties.namespace}:sessions:$sessionId")
                    null
                }
            }
        }

Expected behavior
The custom mapper above should compile in Kotlin.

Sample

N/A

Start in spring-session-data-redis/src/main/java/org/springframework/session/data/redis/RedisIndexedSessionRepository.java at setRedisSessionMapper and getSession. Check the mapper's nullability against the nullable result handled by getSession, then verify that the Kotlin custom mapper in the issue compiles while invalid sessions can still return null.