Face Nodes not represented properly for MPAS meshes with missing values

[philipc2]

Overview

When converting an MPAS mesh to the UGRID conventions, grid cells that contain missing values (which are replaced with INT_FILL_VALUE can lead to geometries with less than 3 nodes which can not be considered faces, as shown in the figure below.

MCVE

Dataset Used: oQU480.230422.nc

mpas_root_filepath = "../../test/meshfiles/mpas/"
mpas_dataset_filepath = mpas_root_filepath + "other/oQU480.230422.nc"

uxgrid = ux.open_grid(mpas_dataset_filepath, use_dual=True)
uxds = ux.UxDataset(uxgrid=uxgrid)

# investigating face nodes showcases the invalid faces
uxds.uxgrid.Mesh2_face_nodes.values