fixed mesh_to_compas returning empty Mesh

[chenkasirer]

Fixed mesh_to_compas returning an empty Mesh when colors and/or face normals are missing.

Reported by @jckenny59. When attempting to convert a GH mesh to compas one got a Mesh with no vertices or faces.

from compas_rhino.conversions import  mesh_to_compas
import Rhino.Geometry as rg

joint_mesh = rg.Mesh()
mesh = rg.Mesh.CreateFromBrep(brep.native_brep, rg.MeshingParameters.Minimal)
joint_mesh.Append(mesh)

compas_mesh = mesh_to_compas(joint_mesh)
print(compas_mesh)

Would result in

<Mesh with 0 vertices, 0 faces, 0 edges>

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)

[tomvanmele]

should we not do it the other way around? if the normals are not there, don't store them?