Don't skip empty arguments in build commands

[Julow]

On top of https://github.com/tweag/opam-nix/pull/76 but also fix the handling of optional arguments.

The calling convention becomes quite heavy but seems to work. It looks like this:

args=()
args+=("abc")
if foo; then args+=("$(bar)"); fi
"${args[@]}

It's totally possible to use a shorter form when the command do not contain an optional argument but I figure that could also hide some bugs so I kept it simple.

This works for compiling my project, though my project compiled fine with https://github.com/tweag/opam-nix/pull/76 already. How to test this more reliably ?

[balsoft]

I feel like this should work. I'll run it through the whole test suite.

[balsoft]

Seems to work fine on existing test cases. Thanks!

[Julow]

Thanks!