tests/resource/aws_api_gateway_deployment: Fix TestAccAWSAPIGatewayDeployment_StageName_EmptyString check (#11691)
tests/resource/aws_api_gateway_deployment: Fix TestAccAWSAPIGatewayDeployment_StageName_EmptyString check (#11691)

Previously in the acceptance testing:

```
--- FAIL: TestAccAWSAPIGatewayDeployment_StageName_EmptyString (449.53s)
    testing.go:640: Step 0 error: Check failed: Check 2/2 error: aws_api_gateway_deployment.test: Attribute 'stage_name' found when not expected
```

If I recall correctly, this check was being swapped back-and-forth in the Terraform 0.12 acceptance testing framework when the Terraform 0.11 shims were being implemented. We landed on supporting the old empty string implementation (the new implementation here) rather than forcing all similar test checks being updated across all Terraform Providers, however this particular instance was never reverted and is needlessly failing.

Output from acceptance testing:

```
--- PASS: TestAccAWSAPIGatewayDeployment_StageName_EmptyString (25.78s)
```
kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws