Calling the shape function gradients and divergences for Nédélec variables return zeros

[csdechant]

Bug Description

It seems that due to the shape function gradients and divergences not being computed for HCurl elements within libMesh (see warnings in libMesh file HCurl elements), MOOSE returns zero entries when calling gradSln(), adGradSln(), and divSln() for Nédélec variables.

The inclusion of an error message for these functions is not straight forward, since VectorKernel.C and ADVectorKernel.C defines the _grad_u using gradSln() and adGradSln(), respectively. This mean if an error message were to be introduced at the gradSln() and adGradSln() level, the error would be thrown whenever a Nédélec variables is used with objects inheriting from VectorKernel.C or ADVectorKernel.C.

A warning message could be added with using Nédélec variables, but a blanket warning message for a whole variable family type seems unusual.

Steps to Reproduce

This behavior can be observed by simply using a Nédélec variable within the test file test/tests/kernels/ad_transient_diffusion/ad_transient_vector_diffusion.i (please make sure to edit the mesh element type to something that is compatible for Nédélec variables, such as elem_type = TRI6). The resulting output file will show no diffusion for the solution variable.

Impact

Users currently encounter incorrect solution outputs when using Nédélec variables for diffusion or divergence based problem types without warning.

[Optional] Diagnostics

No response