check condition for whether a workflow set is fitted is no longer correct

[simonpcouch]

  if (!"result" %in% colnames(candidates)) {
    cli_abort(
      "The supplied workflow_set must be fitted to resamples with 
       workflows::workflow_map() before being added to a data stack.",
      call = caller_env(0)
    )
  }

with unfitted workflowset, we see:

stacks() %>% add_candidates(wf_set)
#> Error in `.f()`:
#> ! The second argument to add_candidates() should inherit from one of `tune_results` or `workflow_set`, but its class is list.

The result column is now initialized, but left as empty tibbles, in workflow_set(). Related to #120.

[github-actions]

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