fix(archlinux): avoid nonetype error

Author: noelmcloughlin

Diff for: postgres/osfamilymap.yaml

@@ -62,7 +62,7 @@ RedHat:
     gpgkey: 'https://download.postgresql.org/pub/repos/yum/RPM-GPG-KEY-PGDG-{{ release }}'
     baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/redhat/rhel-$releasever-$basearch'
 
-{%- if grains.get('osmajorrelease') >= 7 %}
+{%- if grains.get('osmajorrelease', 0) >= 7 %}
   pkg_python: python3-psycopg2
 {%- endif %}