vkprove returns pased:0 failed:0 when there is no plan or an empty file

[ebo]

This should return a warning or error and not pretend that it is a valid plan.

[ebo]

I have not found anything in the specs to define the todo behaviour of vkprove. I did find the following in Perl's prove documentation:

"... the failing test that has been marked as TODO does not interfere with the otherwise successful test-run..."

So the final results for vkprove appear to be correct. Should add this note in the docs.