resource/aws_s3_bucket: Prevent panic with empty replication_configuration rules filter
resource/aws_s3_bucket: Prevent panic with empty replication_configuration rules filter

References:

* https://github.com/terraform-providers/terraform-provider-aws/issues/7427
* https://github.com/terraform-providers/terraform-provider-aws/issues/6600

This is a best effort fix given this may be caused by a manual console update and we do not have a replicating Terraform configuration. The previous code was missing a `nil` check before type assertion.

Output from acceptance testing:

```
--- PASS: TestAccAWSS3Bucket_shouldFailNotFound (17.44s)
--- PASS: TestAccAWSS3Bucket_Cors_EmptyOrigin (29.29s)
--- PASS: TestAccAWSS3Bucket_enableDefaultEncryption_whenAES256IsUsed (29.94s)
--- PASS: TestAccAWSS3Bucket_Cors_Delete (31.64s)
--- PASS: TestAccAWSS3Bucket_importBasic (33.49s)
--- PASS: TestAccAWSS3Bucket_ReplicationExpectVersioningValidationError (41.70s)
--- PASS: TestAccAWSS3Bucket_Logging (47.80s)
--- PASS: TestAccAWSS3Bucket_LifecycleExpireMarkerOnly (52.04s)
--- PASS: TestAccAWSS3Bucket_disableDefaultEncryption_whenDefaultEncryptionIsEnabled (52.96s)
--- PASS: TestAccAWSS3Bucket_Cors_Update (53.36s)
--- PASS: TestAccAWSS3Bucket_objectLock (53.81s)
--- PASS: TestAccAWSS3Bucket_enableDefaultEncryption_whenTypical (58.86s)
--- PASS: TestAccAWSS3Bucket_WebsiteRoutingRules (48.75s)
--- PASS: TestAccAWSS3Bucket_ReplicationWithoutStorageClass (70.75s)
--- PASS: TestAccAWSS3Bucket_Policy (70.96s)
--- PASS: TestAccAWSS3Bucket_ReplicationWithoutPrefix (72.03s)
--- PASS: TestAccAWSS3Bucket_Versioning (72.13s)
--- PASS: TestAccAWSS3Bucket_Lifecycle (73.01s)
--- PASS: TestAccAWSS3Bucket_RequestPayer (47.45s)
--- PASS: TestAccAWSS3Bucket_basic (24.91s)
--- PASS: TestAccAWSS3Bucket_namePrefix (24.41s)
--- PASS: TestAccAWSS3Bucket_region (31.70s)
--- PASS: TestAccAWSS3Bucket_generatedName (26.78s)
--- PASS: TestAccAWSS3Bucket_acceleration (49.96s)
--- PASS: TestAccAWSS3Bucket_importWithPolicy (35.90s)
--- PASS: TestAccAWSS3Bucket_UpdateAcl (48.55s)
--- PASS: TestAccAWSS3Bucket_WebsiteRedirect (70.34s)
--- PASS: TestAccAWSS3Bucket_Website_Simple (70.38s)
--- PASS: TestAccAWSS3Bucket_ReplicationConfiguration_Rule_Destination_AccessControlTranslation (148.13s)
--- PASS: TestAccAWSS3Bucket_ReplicationSchemaV2 (216.56s)
--- PASS: TestAccAWSS3Bucket_Replication (225.08s)
```

kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws