Crash when leaf nodes have no samples

[taltstidl]

Thanks for the great library. The visualizations are truly great. However, we're running into a crash when some of the leaf nodes do not have any samples in the X_train and y_train data. What seems to happen is the following:

• When calling render = viz_model.view() all necessary legend/node/leaf files are created. However, as some of the leaf nodes have no samples, the check in https://github.com/parrt/dtreeviz/blob/a3d02a5fd382bb8e70af92cc9110d9ef73bc86a0/dtreeviz/trees.py#L1261 comes into play and no .svg file is generated for that leaf.
• Later when calling render.save('file.svg'), dot tries to render the SVG, but fails as the dot code returned by view still references the SVG files even though they were not created.

Two possible solutions to this:

1. https://github.com/parrt/dtreeviz/blob/a3d02a5fd382bb8e70af92cc9110d9ef73bc86a0/dtreeviz/trees.py#L1240 Here, render an empty SVG or similar so it can later be rendered (this would be my preferred option).
2. https://github.com/parrt/dtreeviz/blob/a3d02a5fd382bb8e70af92cc9110d9ef73bc86a0/dtreeviz/trees.py#L607 Here, do not add a leaf if no file could be created (would likely need to add a suitable return value to _class_leaf_viz).

[tlapusan]

Hi @taltstidl ,

Thanks for raising this issue. There was some work related to it but it wasn't done completely. ex. https://github.com/parrt/dtreeviz/pull/299

I will take a look on it in the next days, it seems to be an issue also when the split nodes don't have samples.