Disallow empty string in package_apt_pin

[wyardley]

• Require package_apt_pin to be a non-empty string or number
• Use nil instead of empty string in test for empty pin

Resolves a test failure that seems to have cropped up since the last time tests ran (probably a new version of the apt module).

This is technically breaking in a minor way, though guessing we don't need to cut a major for it. Alternately, I could continue to be less strict in the module's validation, and just update the test case 🤷

  3) rabbitmq on ubuntu-20.04-x86_64 with no pin it sets up an apt::source is expected to contain Apt::Source[rabbitmq] with location => "https://packagecloud.io/rabbitmq/rabbitmq-server/ubuntu", repos => "main" and key => "{\"id\"=>\"8C695B0219AFDEB04A058ED8F4E789204D206F89\", \"source\"=>\"https://packagecloud.io/rabbitmq/rabbitmq-server/gpgkey\", \"content\"=>nil}"
     Failure/Error:
       is_expected.to contain_apt__source('rabbitmq').with(
         'location' => "https://packagecloud.io/rabbitmq/rabbitmq-server/#{os_facts['os']['name'].downcase}",
         'repos' => 'main',
         'key' => '{"id"=>"8C695B0219AFDEB04A058ED8F4E789204D206F89", "source"=>"https://packagecloud.io/rabbitmq/rabbitmq-server/gpgkey", "content"=>nil}'
       )
     
     Puppet::PreformattedError:
       Evaluation Error: Error while evaluating a Resource Statement, Apt::Pin[rabbitmq]: parameter 'priority' expects a value of type Integer or String[1], got String (file: /Users/wby/git/puppet-rabbitmq/spec/fixtures/modules/rabbitmq/manifests/repo/apt.pp, line: 41) on node xxxx