Plotting fails when no variable params

[chanhosuh]

When there is only one iteration from ParametrizedPoolIterator, the following error occurs:

Traceback (most recent call last):
  File "/home/runner/work/curvesim/curvesim/.github/workflows/run_sim_request.py", line 33, in <module>
    sim_results.plot(save_as="./results/plots.html")
  File "/home/runner/work/curvesim/curvesim/./curvesim/metrics/results/sim_results.py", line 120, in plot
    return self.plotter.plot(self, summary, data, save_as)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/result_plotter.py", line 54, in plot
    chart = self.plot_results(results)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/result_plotter.py", line 15, in plot_results
    summary = plot_summary(results)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/result_plotter.py", line 29, in plot_summary
    page = make_page_from_results(results, data_key, axes)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/make_page.py", line 46, in make_page_from_results
    return make_page(data_dict, config, factor_keys, axes["metric"], selectors)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/make_page.py", line 103, in make_page
    subplot = make_subplot(cfg, metrics, factors, metric_axis, kwargs)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/make_page.py", line 1[70](https://github.com/curveresearch/curvesim/actions/runs/5632213294/job/15259937175#step:6:71), in make_subplot
    return make(config, metrics, metric_axis, factors, **kwargs)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/make_page.py", line 197, in make_single_chart
    tooltip = make_tooltip(chart["encoding"], metric_axis, factors)
  File "/home/runner/work/curvesim/curvesim/./curvesim/plot/altair/results/tooltip.py", line 7, in make_tooltip
    if "timestamp" in encoding["x"]["shorthand"]:
TypeError: argument of type 'UndefinedType' is not iterable

It may be some of the changes I made to fix the failure when there are no variable params and only fixed params are interfering, but they shouldn't(!) since the fix was to just return the pool configured by the fixed params.