Do not create empty enums.

Mac build issued a warning for enum{}.

Change-Id: I9cab958538ac3cec61aeae289d1da0118ecac15f
Tested: Mac build no longer issues warnings.

src/idl_gen_cpp.cpp

@@ -260,6 +260,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
   code += " FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table";
   code += " {\n";
   // Generate field id constants.
+  if (struct_def.fields.vec.size() > 0) {
     code += "  enum {\n";
     for (auto it = struct_def.fields.vec.begin();
          it != struct_def.fields.vec.end();
@@ -271,6 +272,7 @@ static void GenTable(const Parser &parser, StructDef &struct_def,
       }
     }
     code += "  };\n";
+  }
   // Generate the accessors.
   for (auto it = struct_def.fields.vec.begin();
        it != struct_def.fields.vec.end();