fix(MeshHelper): get_triangles on meshes without indices

[nichinichisou0609]

MeshHandler.copy_from_mesh copies mesh.m_IndexBuffer only when it is non-empty, so for a mesh without indices self.m_IndexBuffer stays None, and get_triangles stops at assert self.m_IndexBuffer is not None. Exporting such a mesh (mesh.export()) therefore fails with an AssertionError.

Unity serializes meshes like this, for example:

• a placeholder mesh with vertices but no submeshes (seen with the prefab mesh of a Spine skeleton: 4 vertices, empty index buffer, m_SubMeshes == []);
• an empty mesh with one submesh whose indexCount is 0.

Changes:

• get_triangles: when there is no index buffer, return an empty triangle list for each submesh. If a submesh declares indices (indexCount > 0) while the mesh has no index buffer, raise a ValueError instead, since that data is inconsistent. This matches AssetStudio's GetTriangles, which loops over the submeshes of an empty index buffer without output and fails with an index error when a submesh has indices that the buffer does not hold.
• copy_from_mesh is unchanged, and so are compressed meshes, whose indices come from m_CompressedMesh; the new check only runs when there is no index buffer at all.
• tests/test_mesh_helper.py covers the three cases on the xinzexi_2_n_tex sample mesh, with its fields changed in memory: no submeshes and no indices (empty result, OBJ export with vertices and no faces), submeshes with indexCount 0, and submeshes with indices but no index buffer (ValueError). All three fail on master with the AssertionError.

This is the index-buffer counterpart of the empty VertexData handling added in #366.