Fixes: #14530
When we found an update of IPv6 cidr block, we first tried to
disassociate the old cidr block association. This caused errors if there
was none, it threw an error, as we passed an empty associationId:
```
* aws_subnet.public_subnet.0: InvalidSubnetCidrBlockAssociationId.Malformed: The subnet CIDR block with association ID is malformed
status code: 400, request id: f438f468-9ca4-4000-ba78-63a0f25d390a
* aws_subnet.public_subnet[1]: 1 error(s) occurred:
```
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSubnet_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 18:28:45 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSubnet_ -timeout 120m
=== RUN TestAccAWSSubnet_importBasic
--- PASS: TestAccAWSSubnet_importBasic (55.40s)
=== RUN TestAccAWSSubnet_basic
--- PASS: TestAccAWSSubnet_basic (53.62s)
=== RUN TestAccAWSSubnet_ipv6
--- PASS: TestAccAWSSubnet_ipv6 (125.87s)
=== RUN TestAccAWSSubnet_enableIpv6
--- PASS: TestAccAWSSubnet_enableIpv6 (88.88s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 323.787s
```
8b7f17c039