Check for workdirPath null in Repository.Clone

If you try to call `Repository.Clone` with `null` for `workdirPath`, it throws an `AccessViolationException` somewhere down the line. It took me a while to figure out why my code wasn't working until I discovered a typo of my own, which caused me to pass `null` instead of an actual valid path.

It's really silly and really simple, but I figured throwing an `ArgumentNullException` instead of an `AccessViolationException` wouldn't hurt, right?

*Disclaimer: I have no experience when it comes to contributing to anything that isn't my own, so I hope this is okay.*