SubscriptionManager#subscribed_products fails to parse blank date

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'
```