C++ naive codegen doesn't support empty stencils

[BenWeber42]

@stencil
def simple_example_stencil(a: Field[Edge]):
    with levels_upward:
        pass

leads to

intro_cxx-naive.cpp:27:12: error: expected ‘;’ at end of member declaration
   27 |   void run() ) {}
      |            ^
      |             ;
intro_cxx-naive.cpp:27:14: error: expected unqualified-id before ‘)’ token
   27 |   void run() ) {}
      |              ^

Relevant generated code:

  simple_example_stencil(const ::dawn::mesh_t<LibTag> &mesh, int k_size,
                         ::dawn::edge_field_t<LibTag, ::dawn::float_type> &a) {}

  void run() ) {}
};