fix: sg arn output breaks when providing security groups IDs

[pcn]

what

Currently a join() is run over what is assumed to be all of the created security groups created by the module.aws_security_group. This join fails when there are no security groups being created. What's more, my reading is that only one security group will ever be created, so only one security group ID should ever need to be extracted, so this is probably a leftover from older logic.

why

When using create_security_group = false and associated_security_groups = [<something>] , the module fails with

│ Invalid value for "lists" parameter: element 0 is null; cannot concatenate
│ null values.

This breaks a documented supported configuration (that we happen to use and be blocked by).

references

This is just putting the fix from https://github.com/dmitrijn/terraform-aws-elasticache-memcached per the request in #56 (comment) to get this fix into main.

[pcn]

@Gowiem ping, can you see if this can be tested and released?

[Gowiem]

/terratest

[Gowiem]

/terratest

[Gowiem]

Closes #56