Empty state mismatch

[MTCam]

This appears to be the problem causing this MIRGE-Com issue. There is a reproducing snippet in this MIRGE-Com PR.

Here's the gist:

bnd_discr = discr.discr_from_dd(btag)
bnd_nodes = thaw(actx, bnd_discr.nodes())
bnd_normal = thaw(actx, discr.normal(btag))
result = bnd_nodes @ bnd_normal

The bnd_nodes @ bnd_normal operation fails when the boundary represented by btag has 0 points. (Seems like an odd thing - but this happens often in parallel runs as not all partitions own points on all boundaries). The failure seems due to this difference in the data structures:

bnd_nodes=array([DOFArray((cl.Array([], shape=(0, 3), dtype=float64),)),
       DOFArray((cl.Array([], shape=(0, 3), dtype=float64),)),
       DOFArray((cl.Array([], shape=(0, 3), dtype=float64),))],
      dtype=object)
bnd_normal=array([DOFArray(()), DOFArray(()), DOFArray(())], dtype=object)

[inducer]

This snippet reproduces the problem without MPI.