Permit an absent preferences pillar data key

[boltronics]

The preferences.sls file has a line like:
{% set preferences = apt.get('preferences', apt_map.preferences) %}

In my pillar sls, I had an if condition such as:

{%- if oscodename == 'jessie' %}
  preferences:
    python-tornado:
      package: python-tornado
      pin: release a=jessie-backports
      priority: 950
      explanation:
        - Dependency for salt-minion.
{%- endif %}

When instances were upgraded to stretch, the preference key was no longer used which resulted in Data failed to compile: errors when trying to run highstate (since apt.preferences is still listed there).

In theory, I could just update top.sls to also have the same if condition around the apt.preferences reference, however that approach would lead to unnecessary code duplication - particularly as more if conditions need to be added for different os releases. Instead, I propose here we set a default preferences key with the value of an empty dict so the state can complete without error but still essentially do nothing.

[javierbertoli]

thanks guys!

[aboe76]

@javierbertoli and @boltronics thanks for this.!