message: Fixed empty structs generating bad constructor.
  This was fixed previously here:
  but somehow got undone in intermediate refactors.
diff: @@ -2497,6 +2497,7 @@ class CppGenerator : public BaseGenerator {
  }
  }
  + if (!arg_list.empty()) {
  code_.SetValue("ARG_LIST", arg_list);
  code_.SetValue("INIT_LIST", init_list);
  code_ += " {{STRUCT_NAME}}({{ARG_LIST}})";
  @@ -2512,6 +2513,7 @@ class CppGenerator : public BaseGenerator {
  }
  }
  code_ += " }";
  + }
  // Generate accessor methods of the form:
  // type name() const { return flatbuffers::EndianScalar(name_); }
  