Fix the error when "create_security_group" is set to false

[legal90]

If the create_security_group var is set to false, then aws_security_group.this[0] is not defined, which causes errors (reproduced on terraform v0.12.12):

../terraform-aws-rds-aurora/outputs.tf line 57, in output "this_security_group_id":
  57:   value       = aws_security_group.this[0].id
    |----------------
    | aws_security_group.this is empty tuple

../terraform-aws-rds-aurora/main.tf line 48, in resource "aws_rds_cluster" "this":
  48:   vpc_security_group_ids              = compact(concat([aws_security_group.this[0].id], var.vpc_security_group_ids))
    |----------------
    | aws_security_group.this is empty tuple

This PR fixes that in a similar way how it's done in https://github.com/terraform-aws-modules/terraform-aws-vpc

[legal90]

@antonbabenko Could you please review this PR?

[antonbabenko]

Thanks, Mikhail!

I've just released v2.7.0 of this module.

[github-actions]

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