Workflow visualization has non-intuitive error for empty workflow

To repro:

```
workflow <- DAGWorkflow$new(name="myWorkflow")

workflow$visualize()
```

Outputs:

```
Error in data.frame(id = 1:n, type = type, label = label, stringsAsFactors = FALSE) : 
  arguments imply differing number of rows: 2, 0
```