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.