Handle nil value for AutoVerifiedAttributes in CognitoUserPool

[louy]

Fixes #3009

[bflad]

@luoy thanks for raising this PR! This looks like a good fix, however this change will break this test: https://github.com/terraform-providers/terraform-provider-aws/blob/master/aws/resource_aws_cognito_user_pool_test.go#L277

With this PR, that line should be:
resource.TestCheckResourceAttr("aws_cognito_user_pool.pool", "auto_verified_attributes.#", "0")

Are you able to run the acceptance tests? If so, all these d.Set() with flattenStringList can be adjusted the same way in the code and acceptance testing. If not, no big deal, just let us know and we'll patch/verify it for you.

[Puneeth-n]

@bflad This PR seems to be orphaned. Would you mind if I hijacked it and took it till the end?

[louy]

@Puneeth-n I've not yet managed to find time to finish it so please do!

[Puneeth-n]

Will give it a spin later and create a separate PR. Thanks :)

[bflad]

The updated PR has been merged into master and will release with v1.12.0 of the AWS provider, likely in about a week. 😄

[bflad]

The updated PR has been released in version 1.12.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!