Do not fail for empty reductions

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
```