Calling Hadrian with empty argument fails
$ hadrian/build.cabal.sh ""
Error when running Shake build system:
at want, called at src/Main.hs:89:30 in main:Main
* Depends on: .
* Raised the exception:
Build system error - expected a file, got a directory:
Directory: .
Probably due to calling 'need' on a directory. Shake only permits 'need' on files.