provider/aws: Defensively code around `db_security_group` ingress rules (#7893)
provider/aws: Defensively code around `db_security_group` ingress rules (#7893)

Fixes #7812

All of the options of `aws_db_security_group` ingress rules are
optional. Therefore, when one of them isn't set (and AWS doesn't
calculate the value), Terraform threw a panic

This commit just defensively codes around this fact. It checks to make
sure there is a value returned from the API before adding it to the map

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBSecurityGroup_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSDBSecurityGroup_ -timeout 120m
=== RUN   TestAccAWSDBSecurityGroup_basic
--- PASS: TestAccAWSDBSecurityGroup_basic (38.66s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    38.682s
```
nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws