GitHub dependency doesn't get generated if subdir is not set

[poscat0x04]

for example this will generate cabal-helper.nix

- github: jneira/cabal-helper
   commit: ffb1f57a5ffc6b7ac3c46a9974c4420a6d2bb9b2
   subdirs:
     - .

whereas this wont

- github: jneira/cabal-helper
   commit: ffb1f57a5ffc6b7ac3c46a9974c4420a6d2bb9b2