Mapping of null/blank with optional/required modifier not deterministic in xosgenx

In the `map_xproto_to_django` code in this revision:

https://gerrit.opencord.org/gitweb?p=xos.git;a=blob;f=lib/xos-genx/xosgenx/jinja2_extensions/django.py;h=724b85dc87ac319b0df47a30aefcbba08827b426;hb=HEAD#l85

Depending on the order of iterating over the options list, giving a `null` or `blank` option might be set after the default values for those fields are set by the modifiers - `required` should set both null and blank to False, and `optional` sets both to True.

Fix this, and address the use of null/blank as it relates to optional/required field modifiers.