Fixed empty structs generating bad constructor.

This was fixed previously here:
https://github.com/google/flatbuffers/commit/5fd0fefab644caa2bb395cda79fbee6103035818
but somehow got undone in intermediate refactors.

Change-Id: I86e45a3f96f67a2b3d84d44081403baef6798921

src/idl_gen_cpp.cpp

@@ -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_); }