Correct initial value for the default case

[martinfantini]

Correct the generation of the storage value for the set and enums.

For the definition:

  <template name="Test_3" id="3">
    <uInt32 name="field3" id="11"><copy/></uInt32>
    <field name="TradeCondition" id="277" presence="optional">
      <type name="TradeConditionSet">
        <default/>
      </type>
    </field>
  </template>

There is no set or enum value by default.

And storage definition has to be:

const static TradeConditionSet::instruction_type
Test_3__TradeCondition_instruction(
  operator_default,
  presence_optional,
  277, // id
  "TradeCondition", // name
  "", // ns
  nullptr,  // opContext
  int_value_storage<uint64_t>(), // initial_value
  TradeConditionSet::instruction()->elements(), // element names
  TradeConditionSet::instruction()->num_elements(),// num elements
  nullptr, // ref_instruction
  nullptr, // cpp_ns
  mfast::instruction_tag(0)); // tag

With the int_value_storage<uint64_t>(), // initial_value as default constructor.

This error is in sets and enums.