Do not fail for empty reductions

[inducer]

Originally raised by @MTCam in #264:

A second problem is that this pattern fails to actually execute for CV objects since we have:
CV = [DOFArray, DOFArray, DOFArray, ... , numpy.ndarray = [] ], i.e., an empty array for the species_mass component for non-mixture CV. Attempting a reduction operation using a single gas CV now produces this error:

        if not isinstance(vec, DOFArray):
>           return max(
                nodal_max_loc(dcoll, dd, comp, initial=initial)
                for _, comp in serialize_container(vec)
            )
E           ValueError: max() arg is an empty sequence

../../grudge/grudge/reductions.py:257: ValueError

[inducer]

Making empty reductions work has been the subject of ongoing work for a while, mostly by @majosm, cf:

• Use arraycontext reductions in reductions #199
• Allow empty sub-containers in reductions arraycontext#129

These look a bit stalled ATM, but I think with a push, we can get them across the finish line.