fix(codegen): Fix code generated with headers using --force-pointers

[TheSadlig]

Related to #262:
Some of the generated code (toBrokerMessage(), brokerMessasgeTo*()) was invalid following the introduction (by me) of the --force-pointers flag., whenever headers are present in the contract.

A few notes:

• The isFieldPointer had to be changed to circumvent an issue where some fields' IsRequired is true, but the field is not in the parent's list of required field. I decided not to investigate inside the parser, for obvious reasons.
• I've added a nil check on toBrokerMessage on fields that are required. That same check is not really possible on brokerMessageTo* without checking missing fields afterwards. Since there are currently no validations, I didn't see it necessary at this stage

Please let me know if you have any questions

Thanks,
TheSadlig

[TheSadlig]

Hi @lerenn!
Did you have any chance to take a look at this?
Thanks!