Previously the check for if we are setting `CidrBlock` or `IPv6CidrBlock` during an `Update` of the `aws_network_acl` resource would populate the input struct with a nil string value `""`. This caused our acceptance tests to fail, and broke the resource's functionality if a user only set `CidrBlock` or `IPv6CidrBlock` for either an `ingress` or `egress` rule as the API would error out with an `Invalid CidrBlock` error.
Previously:
```
aws_network_acl.bond: Error creating egress entry: InvalidParameterValue: CIDR block is malformed
status code: 400, request id: 0620e0b7-4e30-4c14-9a7a-5d373cc9f33b
```
Currently:
```
$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSNetworkAcl'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/15 15:41:17 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSNetworkAcl -timeout 120m
=== RUN TestAccAWSNetworkAcl_importBasic
--- PASS: TestAccAWSNetworkAcl_importBasic (26.96s)
=== RUN TestAccAWSNetworkAclRule_basic
--- PASS: TestAccAWSNetworkAclRule_basic (23.08s)
=== RUN TestAccAWSNetworkAclRule_ipv6
--- PASS: TestAccAWSNetworkAclRule_ipv6 (26.24s)
=== RUN TestAccAWSNetworkAcl_EgressAndIngressRules
--- PASS: TestAccAWSNetworkAcl_EgressAndIngressRules (25.11s)
=== RUN TestAccAWSNetworkAcl_OnlyIngressRules_basic
--- PASS: TestAccAWSNetworkAcl_OnlyIngressRules_basic (31.82s)
=== RUN TestAccAWSNetworkAcl_OnlyIngressRules_update
--- PASS: TestAccAWSNetworkAcl_OnlyIngressRules_update (48.59s)
=== RUN TestAccAWSNetworkAcl_OnlyEgressRules
--- PASS: TestAccAWSNetworkAcl_OnlyEgressRules (25.48s)
=== RUN TestAccAWSNetworkAcl_SubnetChange
--- PASS: TestAccAWSNetworkAcl_SubnetChange (57.12s)
=== RUN TestAccAWSNetworkAcl_Subnets
--- PASS: TestAccAWSNetworkAcl_Subnets (67.55s)
=== RUN TestAccAWSNetworkAcl_ipv6Rules
--- PASS: TestAccAWSNetworkAcl_ipv6Rules (31.52s)
=== RUN TestAccAWSNetworkAcl_espProtocol
acc--- PASS: TestAccAWSNetworkAcl_espProtocol (24.37s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 387.855s
```
Jake Champlin