resource/aws_autoscaling_policy: Replace GetOk with GetOkExists

[atsushi-ishibashi]

Fixed: #2528
When you specify 0, GetOk returns false as exists so replaced with GetOkExists.

[radeksimko]

@atsushi-ishibashi
This looks like a reasonable patch 👍 Can you also add a test case which verifies it (i.e. config where the affected fields are undefined)?

Thanks.

[atsushi-ishibashi]

TF_ACC=1 go test ./aws -v -run=TestAccAWSAutoscalingPolicy_zerovalue -timeout 120m
=== RUN   TestAccAWSAutoscalingPolicy_zerovalue
--- PASS: TestAccAWSAutoscalingPolicy_zerovalue (115.46s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	115.487s

I got the below error so added validation.IntAtLeast(1) as ValidateFunc.

aws_autoscaling_policy.foobar_step: Error putting scaling policy: ValidationError: If specified, MinAdjustmentMagnitude must be greater than zero.

[radeksimko]

I was tempted to say that we're not really testing the new behaviour, because AWS API just doesn't return the default value (300), but nil instead, which d.Set() then turns back into 0.
This made me think we could only call d.Set() if the field isn't nil, but that might break some people's configs where they're referencing the field in conditionals, so it's probably ok to leave it as is. 🤷‍♂️

I just rebased your branch and resolved conflicts for you, so we can 🚢 this.

Thanks for the PR, as always! 👍

[bflad]

This has been released in version 1.10.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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