Make nodal reductions work with empty meshes (#256)

* support empty groups in reductions

* simplify conditional

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* don't set a default initial value in nodal_min/nodal_max

* test with and without initial value

* Revert "don't set a default initial value in nodal_min/nodal_max"

This reverts commit 771fb7bab3a18e64248fe7725f53c77a48ef6ae6.

decided against this; not setting an initial value means that initial
value always needs to be passed in multi-volume case

* add test case for empty + no initial value

Co-authored-by: Andreas Klöckner <inform@tiker.net>