How to conditionally add keys to the map in Terraform

Use Ternary expression and merge() like so

resource "aws_cloudformation_stack" "tetra_resources" {
  name = "${local.name_prefix}-tetra-deployer-resources"
 
  template_body = jsonencode({
    Resources = merge(
      {
        for pipeline in local.pipelines : pipeline["name"] => {
          Type = "AWS::CloudFormation::CustomResource"
          Properties = merge({
            # The lambda function that will be invoked to deploy the pipeline
            ServiceToken = ...
            Config       = ...
            },
            # If dataweave is provided, then add the dataweave file to the template
            lookup(pipeline, "dataweave", null) != null ? {
              Dataweave = file("../config/${pipeline["dataweave"]}")
              # Get the last segment of path to get the filename if dataweave is provided
              DataweaveFileName = element(split("/", lookup(pipeline, "dataweave", null)), length(split("/", lookup(pipeline, "dataweave", null))) - 1)
            } : {}
          )
        }
      },
    )
}