Empty-string sub-attribute fails the build for `__structuredAttrs = true`

[ShamrockLee]

Describe the bug

Steps To Reproduce

1. Clone Nixpkgs and change the working directory to its project root.
2. Run

   nix-build --no-out-link --expr '
     (import ./. { }).stdenvNoCC.mkDerivation {
       __structuredAttrs = true;
       name = "test-structured-empty-subattr";
       foo = { "" = true; };
       buildCommand = "touch \$out";
     }
   '

3. See the error.

   this derivation will be built:
     /nix/store/0m0h23b1f09ir63xiq7yckd4rvsr61av-test-structured-empty-subattr.drv
   building '/nix/store/0m0h23b1f09ir63xiq7yckd4rvsr61av-test-structured-empty-subattr.drv'...
   /build/.attrs.sh: line 16: ['']='1': bad array subscript
   error: builder for '/nix/store/0m0h23b1f09ir63xiq7yckd4rvsr61av-test-structured-empty-subattr.drv' failed with exit code 1;
          last 1 log lines:
          > /build/.attrs.sh: line 16: ['']='1': bad array subscript
          For full logs, run:
            nix log /nix/store/0m0h23b1f09ir63xiq7yckd4rvsr61av-test-structured-empty-subattr.drv
   

Expected behavior

The above derivation build.

Metadata

nix-env (Nix) 2.28.5

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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[xokdvium]

This seems like a bash limitation: https://www.gnu.org/software/bash/manual/html_node/Arrays.html

Empty strings cannot be used as associative array keys.