fix issues with handling of empty groups

Author: Matthew Smith

grudge/dt_utils.py

@@ -295,7 +295,9 @@ def dt_geometric_factors(
                 actx.einsum(
                     "fej->e",
                     face_ae_i.reshape(
-                        vgrp.mesh_el_group.nfaces, vgrp.nelements, -1),
+                        vgrp.mesh_el_group.nfaces,
+                        vgrp.nelements,
+                        face_ae_i.shape[-1]),
                     tagged=(FirstAxisIsElementsTag(),))
 
                 for vgrp, face_ae_i in zip(volm_discr.groups, face_areas)
@@ -314,7 +316,9 @@ def dt_geometric_factors(
                 actx.einsum(
                     "fej->e",
                     face_ae_i.reshape(
-                        vgrp.mesh_el_group.nfaces, vgrp.nelements, -1
+                        vgrp.mesh_el_group.nfaces,
+                        vgrp.nelements,
+                        face_ae_i.shape[-1]
                     ) / afgrp.nunit_dofs,
                     tagged=(FirstAxisIsElementsTag(),))
 

grudge/op.py

@@ -928,7 +928,7 @@ def _apply_face_mass_operator(dcoll: DiscretizationCollection, dd_in, vec):
                         surf_ae_i.reshape(
                                 vgrp.mesh_el_group.nfaces,
                                 vgrp.nelements,
-                                -1),
+                                surf_ae_i.shape[-1]),
                         vec_i.reshape(
                                 vgrp.mesh_el_group.nfaces,
                                 vgrp.nelements,