alchemlyb.visualisation.plot_convergence cannot handel the case where the final error is NaN

[xiki-tempula]

If the input to alchemlyb.visualisation.plot_convergence is in such format

        Forward  Forward_Error  Backward  Backward_Error  data_fraction
0 -9.095031e-09    2225.996551  0.000002     3457.639809            0.5
1 -1.334738e-06            NaN -0.000001             NaN            1.0

The function would quit with

---------------------------------------------------------------------------
IndexError                Traceback (most recent call last)
Cell In[29], line 1
----> 1 ax.fill_between([0, 1], np.nan, np.nan)
File ~/miniconda/envs/exs-orionbssfloes/lib/python3.8/site-packages/matplotlib/__init__.py:1442, in _preprocess_data.<locals>.inner(ax, data, *args, **kwargs)
  1439 @functools.wraps(func)
  1440 def inner(ax, *args, data=None, **kwargs):
  1441   if data is None:
-> 1442     return func(ax, *map(sanitize_sequence, args), **kwargs)
  1444   bound = new_sig.bind(ax, *args, **kwargs)
  1445   auto_label = (bound.arguments.get(label_namer)
  1446          or bound.kwargs.get(label_namer))
File ~/miniconda/envs/exs-orionbssfloes/lib/python3.8/site-packages/matplotlib/axes/_axes.py:5431, in Axes.fill_between(self, x, y1, y2, where, interpolate, step, **kwargs)
  5429 def fill_between(self, x, y1, y2=0, where=None, interpolate=False,
  5430         step=None, **kwargs):
-> 5431   return self._fill_between_x_or_y(
  5432     "x", x, y1, y2,
  5433     where=where, interpolate=interpolate, step=step, **kwargs)
File ~/miniconda/envs/exs-orionbssfloes/lib/python3.8/site-packages/matplotlib/axes/_axes.py:5420, in Axes._fill_between_x_or_y(self, ind_dir, ind, dep1, dep2, where, interpolate, step, **kwargs)
  5417 collection = mcoll.PolyCollection(polys, **kwargs)
  5419 # now update the datalim and autoscale
-> 5420 pts = np.row_stack([np.column_stack([ind[where], dep1[where]]),
  5421           np.column_stack([ind[where], dep2[where]])])
  5422 if ind_dir == "y":
  5423   pts = pts[:, ::-1]
File ~/miniconda/envs/exs-orionbssfloes/lib/python3.8/site-packages/numpy/ma/core.py:3254, in MaskedArray.__getitem__(self, indx)
  3249   return None
  3251 if _mask is not nomask:
  3252   # _mask cannot be a subclass, so it tells us whether we should
  3253   # expect a scalar. It also cannot be of dtype object.
-> 3254   mout = _mask[indx]
  3255   scalar_expected = _is_scalar(mout)
  3257 else:
  3258   # attempt to apply the heuristic to avoid constructing a full mask
IndexError: invalid index to scalar variable.