Add helper for argument validation

Each Cake addin requires some kind of argument validation (or at least should :). Instead of having to implement this in every addin it would be nice if this could be provided for common cases (null check, empty string check, ...) by Cake.Core.

Start by reviewing the issue discussion and the Cake.Core area, since no implementation file, test, or entry point is named. Identify the common validation cases and intended helper API from the discussion; done should provide reusable handling for cases such as null and empty-string arguments, with coverage for the agreed behavior.

Written by the indexing model from the issue text.