(maint) Add error message if GitURI is nil

Prior to this commit, if the options param was left out from the
ci:test:git rake task, it would only tell the user that nil was an
unrecognized task i.e.

`#<ArgumentError:  is not recognized.>`

This was not helpful in the case where that uri variable was nil.

This commit changes that by adding a ArgumentError check to see if that
git uri is nil.

I'm not entirely convinced this is the greatest error message either, so if anyone has a better suggestion please let me know.