provider/aws: Change of aws_subnet ipv6 causing update failure

[stack72]

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

[grubernaut]

LGTM

[ghost]

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