fetchFromGitLab: Fix grouping issue for group arg

Issue introduced by c3612b97.

Example output before that commit (witch a random package using
fetchFromGitLab):

$ nix-instantiate --eval -A hiawatha.src.urls
[ "https://gitlab.com/api/v4/projects/hsleisink%2Fhiawatha/repository/archive.tar.gz?sha=v10.8.1" ]

After the commit:

$ nix-instantiate --eval -A hiawatha.src.urls
[ "https://gitlab.com/api/v4/projects/%2Fhsleisink%2Fhiawatha/repository/archive.tar.gz?sha=v10.8.1" ]

And with this very commit, which fixes the issue:

$ nix-instantiate --eval -A hiawatha.src.urls
[ "https://gitlab.com/api/v4/projects/hsleisink%2Fhiawatha/repository/archive.tar.gz?sha=v10.8.1" ]

The problem was that it has used optionalString in conjunction with "+",
so the optionalString only applied to the left-hand side of the +
operator.

Thanks to @ryantm for reporting the issue at:

https://github.com/NixOS/nixpkgs/pull/46487/commits/c3612b97e625ad0d70c6ba56d191bcb9a403f44c#r220576764



Signed-off-by: aszlig <aszlig@nix.build>
Reported-by: Ryan Mulligan <ryan@ryantm.com>