Allow undef return value for dehydrated::file function

[Tiacs]

Pull Request (PR) description

While debugging another issue, I came a cross the following evaluation error in dehydrated::file:

Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Resource Statement, Evaluation Error: Error while evaluating a Function Call, value returned from function 'getfile' has wrong type, expects a String value, got Undef (file: /etc/puppetlabs/code/environments/production/modules/dehydrated/manifests/certificate/collect.pp, line: 47, column: 12) (file: /etc/puppetlabs/code/environments/production/modules/dehydrated/manifests/init.pp, line: 292) on XXXXX

While not being the cause of my original issue, this evaluation error prevented me from getting further debugging information, which I needed to fix my problem.

This Pull Request (PR) fixes the following issues

This pull request fixes the return type of dehydrated::file to also accept undef as return value, which is returned in case of an error.

I believe this is the intended behavior, since it matches the remaining code, handling undef in case of an error.