provider/aws: guard against missing image_digest

[nicolai86]

This PR fixes #7956.

AWS does not add an explicit :latest when no digest is given, which leads to a crash for setups like this.

resource "aws_ecs_task_definition" "mongo" {
  family = "mongodb"
  container_definitions = <<DEFINITION
[
  {
    "cpu": 128,
    "environment": [{
      "name": "SECRET",
      "value": "KEY"
    }],
    "essential": true,
    "image": "mongo",
    "memory": 128,
    "name": "mongodb"
  }
]
DEFINITION
}

This PR fixes this issue.

[stack72]

Hi @nicolai86

Thanks so much fro the bug fix here :)

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEcsDataSource_ecsContainerDefinition'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEcsDataSource_ecsContainerDefinition -timeout 120m
=== RUN   TestAccAWSEcsDataSource_ecsContainerDefinition
--- PASS: TestAccAWSEcsDataSource_ecsContainerDefinition (123.78s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    123.796s

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