[Bug]: Handle visualization of empty graph properly

[thierry-martinez]

Currently, visualizing an empty graph (or an empty pattern) raises an exception.

>>> OpenGraph(nx.Graph(), (), (), {}).draw()
[...]
  File "/home/tmartine/miniforge3/lib/python3.13/site-packages/graphix/visualization.py", line 710, in _
    l_max = max(layers.values())
ValueError: max() iterable argument is empty

>>> Pattern().draw()
[...]
  File "/home/tmartine/miniforge3/lib/python3.13/site-packages/numpy/linalg/_linalg.py", line 144, in _raise_linalgerror_singular
    raise LinAlgError("Singular matrix")
numpy.linalg.LinAlgError: Singular matrix

In #549, the situation will be handled more gracefully by raising a ValueError with a dedicated message. However, it would be preferable to allow visualization of empty graphs without any failures.