Set default value for environment variable when not already set

We'd like to set an environment variable for a pipeline job to inherit from a global environment variable when present, but to default to a reasonable value when not. This was possible in scripted pipeline using the following snippet:

def foo = env.FOO ?: 'foo'
withEnv(["BAR=${foo}"]) {
  // build
}

My attempts to recreate this in declarative pipelines have proven unsuccessful. If this is already possible, please provide some documentation. If it's not possible, please consider this a feature request.