fix: resolve missing target features to 'null'

[tilpner]

I ran into an error where the evaluation of whether a dependency is used was accessing non-existent target values, and failing:

       at /nix/store/41ra1jaz5pn2rddqbf7j5csj0vn5whl8-services-crate2nix/crate/Cargo-generated.nix:4950:58:

         4949|             packageId = "curve25519-dalek-derive";
         4950|             target = { target, features }: ((!("fiat" == target."curve25519_dalek_backend")) && (!("serial" == target."curve25519_dalek_backend")) && ("x86_64" == target."arch"));
             |                                                          ^
         4951|           }

curve25519-dalek uses --cfg (target) values to switch between different cryptographic backends.

This kind of fallback is already present, but only for checking whether a value is defined, not for comparisons:

https://github.com/nix-community/crate2nix/blob/e5372757075fc93c7624131069fa5d22ed88c4fd/crate2nix/src/render.rs#L216

I chose null only as a value to which no string will compare positively, theoretically this could be any non-string value.

The alternative is probably to continue failing loudly, instead of covering up the lack of a cfg value. If that is preferred, a way of setting them globally/per-crate should be documented (I didn't find it), but it might be tedious if many dependencies use a lot of these variables.

[kolloch]

For a moment there, I was a bit confused by the operator precedence there.

Looks good to me, thank you.

[kolloch]

Feel free to add yourself to the CHANGELOG.md in another PR!

[tilpner]

Thanks for the quick review (and your continued maintenance)! :)

If it were a major improvement, or a breaking change, I'd add it to the changelog, but I'm not sure a minor compatibility fix needs to be in there.