Fixed warning "Empty_Rule is defined, but unused" in for 2D and 3D.
requested to merge henrik.stolzmann/dune-uggrid:fix_warning_defined_but_unused_variable_empty_rule into master
I added the attribute "[[maybe_unused]]" to the two definitions of variable "Empty_Rule" to avoid a warning.
The attribute "DUNE_UNUSED" did not work.