fix: exception calling geometry of compas.datastructures.Part`

[gonzalocasas]

If the part has no features assigned, the call to .geometry causes an exception, but additionally, it's very inefficient to do all those transformations from shape to mesh back to polyhedron which, in the case of no attached features to be boolean'ed into the geometry, is not needed.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[gonzalocasas]

/cc @funkchaser

[tomvanmele]

i think this can be further reduced since geometrical shapes can be converted to triangulated polyhedrons directly.

A = self.shape.to_vertices_and_faces(triangulated=True)
for shape in operation in self.features:
    B = shape.to_vertices_and_faces(triangulated=True)
    A = Part.operations[operation](A, B)  # this is always a triangulation...
geometry = Shape(*A)

[gonzalocasas]

done! 510bb43