Add nil checks for route table association state

[blafrisch]

In aws-iso region the AssociationState isn't set in the Associations for route tables. This caused a nil pointer dereference and an RPC error when refreshing state on an aws_route_table_association resource.

For reference the output from the AWS CLI command aws ec2 describe-route-tables looks like this in AWS us-west-2:

{
    "RouteTables": [
        {
            "Associations": [
                {
                    "AssociationState": {
                        "State": "associated"
                    },
                    "Main": false,
                    "RouteTableAssociationId": "rtbassoc-000000000000000000",
                    "RouteTableId": "rtb-000000000000000000",
                    "SubnetId": "subnet-000000000000000000"
                }
...

And looks like this in an AWS ISO region:

{
    "RouteTables": [
        {
            "Associations": [
                {
                    "Main": false,
                    "RouteTableAssociationId": "rtbassoc-000000000000000000",
                    "RouteTableId": "rtb-000000000000000000",
                    "SubnetId": "subnet-000000000000000000"
                }
...

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Output from acceptance testing (chosen because of this PR #21710):

% make testacc PKG_NAME=internal/service/ec2 TESTARGS='-run=TestAccEC2RouteTable_basic\|TestAccEC2RouteTableAssociation_Subnet_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 20  -run=TestAccEC2RouteTable_basic\|TestAccEC2RouteTableAssociation_Subnet_basic -timeout 180m
=== RUN   TestAccEC2RouteTableAssociation_Subnet_basic
=== PAUSE TestAccEC2RouteTableAssociation_Subnet_basic
=== RUN   TestAccEC2RouteTable_basic
=== PAUSE TestAccEC2RouteTable_basic
=== CONT  TestAccEC2RouteTableAssociation_Subnet_basic
=== CONT  TestAccEC2RouteTable_basic
--- PASS: TestAccEC2RouteTable_basic (65.35s)
--- PASS: TestAccEC2RouteTableAssociation_Subnet_basic (78.33s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	82.046s

This is my first PR for this project, let me know if there's anything else I should add/update/change. Thank you!

cc: @YakDriver @ewbankkit because of other recent ISO region related changes and context.

[github-actions]

Welcome @blafrisch 👋

It looks like this is your first Pull Request submission to the Terraform AWS Provider! If you haven’t already done so please make sure you have checked out our CONTRIBUTING guide and FAQ to make sure your contribution is adhering to best practice and has all the necessary elements in place for a successful approval.

Also take a look at our FAQ which details how we prioritize Pull Requests for inclusion.

Thanks again, and welcome to the community! 😃

[ewbankkit]

LGTM 🚀.

% make testacc PKG_NAME=internal/service/ec2 TESTARGS='-run=TestAccEC2RouteTableAssociation_\|TestAccEC2MainRouteTableAssociation_' ACCTEST_PARALLELISM=4
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 4  -run=TestAccEC2RouteTableAssociation_\|TestAccEC2MainRouteTableAssociation_ -timeout 180m
=== RUN   TestAccEC2MainRouteTableAssociation_basic
=== PAUSE TestAccEC2MainRouteTableAssociation_basic
=== RUN   TestAccEC2RouteTableAssociation_Subnet_basic
=== PAUSE TestAccEC2RouteTableAssociation_Subnet_basic
=== RUN   TestAccEC2RouteTableAssociation_Subnet_changeRouteTable
=== PAUSE TestAccEC2RouteTableAssociation_Subnet_changeRouteTable
=== RUN   TestAccEC2RouteTableAssociation_Gateway_basic
=== PAUSE TestAccEC2RouteTableAssociation_Gateway_basic
=== RUN   TestAccEC2RouteTableAssociation_Gateway_changeRouteTable
=== PAUSE TestAccEC2RouteTableAssociation_Gateway_changeRouteTable
=== RUN   TestAccEC2RouteTableAssociation_disappears
=== PAUSE TestAccEC2RouteTableAssociation_disappears
=== CONT  TestAccEC2MainRouteTableAssociation_basic
=== CONT  TestAccEC2RouteTableAssociation_Subnet_changeRouteTable
=== CONT  TestAccEC2RouteTableAssociation_Gateway_basic
=== CONT  TestAccEC2RouteTableAssociation_disappears
--- PASS: TestAccEC2RouteTableAssociation_disappears (35.52s)
=== CONT  TestAccEC2RouteTableAssociation_Gateway_changeRouteTable
--- PASS: TestAccEC2RouteTableAssociation_Gateway_basic (43.95s)
=== CONT  TestAccEC2RouteTableAssociation_Subnet_basic
--- PASS: TestAccEC2RouteTableAssociation_Subnet_changeRouteTable (63.39s)
--- PASS: TestAccEC2MainRouteTableAssociation_basic (63.73s)
--- PASS: TestAccEC2RouteTableAssociation_Subnet_basic (36.79s)
--- PASS: TestAccEC2RouteTableAssociation_Gateway_changeRouteTable (66.05s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	108.221s

[ewbankkit]

@blafrisch Thanks for the contribution 🎉 👏.

[github-actions]

This functionality has been released in v4.0.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.