appengine: Suppress null automatic_scaling blocks during Read

[modular-magician]

Fixes hashicorp/terraform-provider-google#17531

I went with this over O+C or a DSF because the field exists in a oneof (scaling in proto) and those options both preserve the set-but-deeply-nil automatic_scaling block in state, meaning we'd eventually send it to the server even if the user was sending a different scaling entry. This approach is worse if they start returning values for existing fields because it will break users again, but it's at least mitigatable with lifecycle.ignore_changes if that happens.

Release Note Template for Downstream PRs (will be copied)

appengine: added suppression for a diff in google_app_engine_standard_app_version.automatic_scaling when the block is unset in configuration

Derived from GoogleCloudPlatform/magic-modules#10476