SubscriptionManager#subscribed_products fails to parse blank date

[bdunne]

Previous errors looked like this:

ArgumentError: invalid date
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:96:in `strptime'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:96:in `format_values'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:91:in `block in parse_content'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:85:in `each'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:85:in `each_with_object'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:85:in `parse_content'
    from /opt/rh/ruby193/root/usr/share/gems/gems/linux_admin-0.3.0/lib/linux_admin/registration_system/subscription_manager.rb:80:in `parse_output'

[coveralls]

Coverage remained the same when pulling b34944a on brandondunne:subscription_manager_fails_to_parse_blank_date into e00e317 on ManageIQ:master.