I am using AWS CloudFormation to configure EventBridge Rules to direct events to different SQS queues depending on their contents.
For example:

{
    "detail": {
        "data": {...},
        "metadata": {
            "api_version": "1",
            "some_other_field": "whatever"
        }
    }
}

Or:

{
    "detail": {
        "data": {...},
        "metadata": {
            "some_other_field": "whatever"
        }
    }
}

In the first case the event will be sent to the version 1 API, and in the second case the event should also be sent to the version 1 API by default since it is not defined. I should be able to configure this by specifically matching against the null value as specified in the docs, however I can’t use that functionality because CloudFormation does not allow null values anywhere in the stack. So I was going to define the rules like this:

Resources:
    ExampleRule1:
        Type: AWS::Events::Rule
        Properties:
          Description: "..."
          EventBusName: "..."
          EventPattern: 
              detail.metadata:
                - api_version: 1
                - some_other_field: "whatever"
          Name: "..."
          RoleArn: "..."
          State: "..."
          Targets: 
            - "version 1 target"
    ExampleRule2:
        Type: AWS::Events::Rule
        Properties:
          Description: "..."
          EventBusName: "..."
          EventPattern: 
              detail.metadata:
                - api_version: 2
                - some_other_field: "whatever"
          Name: "..."
          RoleArn: "..."
          State: "..."
          Targets: 
            - "version 2 target"
    ExampleRuleDefault:
        Type: AWS::Events::Rule
        Properties:
          Description: "..."
          EventBusName: "..."
          EventPattern: 
              detail.metadata:
                # Remove the version rule so it matches 
                # when the api_version is missing
                - some_other_field: "whatever"
          Name: "..."
          RoleArn: "..."
          State: "..."
          Targets: 
            - "version 1 target"

However this documentation suggests that missing values are treated as "*": "*" which would seem to imply that if someone sent api_version: 2 then there is a possibility that ExampleRuleDefault could capture it. I couldn’t find any reference for what the behavior would be, so I am unsure that this is an acceptable solution.

So my question is, how can I define the rules of to capture events missing that value and not capture other events? (ps. requiring the api_version field is not an option)