Fixed Cognito Lambda Config Validation for optional ARN configurations

[Ninir]

Fixes #2365

[catsby]

👍

[Ninir]

As a sidenote:

make testacc TEST=./aws TESTARGS='-run=TestAccAWSCognitoUserPool_'                            
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCognitoUserPool_ -timeout 120m
=== RUN   TestAccAWSCognitoUserPool_importBasic
--- PASS: TestAccAWSCognitoUserPool_importBasic (21.54s)
=== RUN   TestAccAWSCognitoUserPool_basic
--- PASS: TestAccAWSCognitoUserPool_basic (15.89s)
=== RUN   TestAccAWSCognitoUserPool_withAdminCreateUserConfiguration
--- PASS: TestAccAWSCognitoUserPool_withAdminCreateUserConfiguration (26.60s)
=== RUN   TestAccAWSCognitoUserPool_withDeviceConfiguration
--- PASS: TestAccAWSCognitoUserPool_withDeviceConfiguration (28.54s)
=== RUN   TestAccAWSCognitoUserPool_withEmailVerificationMessage
--- PASS: TestAccAWSCognitoUserPool_withEmailVerificationMessage (27.04s)
=== RUN   TestAccAWSCognitoUserPool_withSmsVerificationMessage
--- PASS: TestAccAWSCognitoUserPool_withSmsVerificationMessage (27.73s)
=== RUN   TestAccAWSCognitoUserPool_withEmailConfiguration
--- PASS: TestAccAWSCognitoUserPool_withEmailConfiguration (26.33s)
=== RUN   TestAccAWSCognitoUserPool_withSmsConfiguration
--- PASS: TestAccAWSCognitoUserPool_withSmsConfiguration (33.68s)
=== RUN   TestAccAWSCognitoUserPool_withSmsConfigurationUpdated
--- PASS: TestAccAWSCognitoUserPool_withSmsConfigurationUpdated (46.65s)
=== RUN   TestAccAWSCognitoUserPool_withTags
--- PASS: TestAccAWSCognitoUserPool_withTags (26.16s)
=== RUN   TestAccAWSCognitoUserPool_withAliasAttributes
--- PASS: TestAccAWSCognitoUserPool_withAliasAttributes (27.45s)
=== RUN   TestAccAWSCognitoUserPool_withPasswordPolicy
--- PASS: TestAccAWSCognitoUserPool_withPasswordPolicy (27.24s)
=== RUN   TestAccAWSCognitoUserPool_withLambdaConfig
--- PASS: TestAccAWSCognitoUserPool_withLambdaConfig (50.29s)
=== RUN   TestAccAWSCognitoUserPool_withSchemaAttributes
--- PASS: TestAccAWSCognitoUserPool_withSchemaAttributes (45.05s)
=== RUN   TestAccAWSCognitoUserPool_withVerificationMessageTemplate
--- PASS: TestAccAWSCognitoUserPool_withVerificationMessageTemplate (34.34s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	464.577s

[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!