The issue is that the same expression (dx0/dr0) appears in
both a user provided expression and the discretization
metrics and the code generator gets confused.

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

Test that normal DOFArrays for empty boundaries have the same number of groups as the corresponding discretization

* uses logger instead of print
* uses the provided `ctx_factory` instead of `create_some_context`