Check `if self.cv is None` in MeshMeta

[jasper-tms]

I had downloaded some meshes previously and I wanted to load them while on the airplane yesterday, and I struggled to figure out how to get MeshMeta.mesh(seg_id=blah) to give me the mesh. I was getting this error:

from meshparty import trimesh_io
mm = trimesh_io.MeshMeta(disk_cache_path='.')
mesh = mm.mesh(seg_id=720575941428754936)  # 720575941428754936.h5 exists in current folder


--> 699     if not isinstance(self.cv.mesh, ShardedMultiLevelPrecomputedMeshSource):
AttributeError: 'NoneType' object has no attribute 'mesh'

So clearly it didn't like that I hadn't set cv_path. I tried setting cv_path to some dummy path gs://foobar but it understandably tried querying that bucket and failed. Ultimately I figured out I could change

MeshParty/meshparty/trimesh_io.py

Line 699 in e84a9e6

if not isinstance(self.cv.mesh, ShardedMultiLevelPrecomputedMeshSource):

to

        if self.cv is None or not isinstance(self.cv.mesh,
                                             ShardedMultiLevelPrecomputedMeshSource):

to get the code snippet above to work. This change is also necessary to get the more direct mm.mesh(filename='720575941428754936.h5') to work when there's no cv_path.

I would imagine this check of self.cv is None needs to be added a few more places to get all use cases to work, but I stopped at this one because it's all I needed, and I thought I'd contribute at least this one change.