fix: use default indexes if non where given by the lockfile

[ruben-arts]

Fixes:

/home/runner/.pixi/bin/pixi install --locked --manifest-path pyproject.toml --color always
  thread 'main' panicked at /home/runner/work/pixi/pixi/src/install_pypi.rs:611:10:
  called `Option::unwrap()` on a `None` value
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: The process '/home/runner/.pixi/bin/pixi' failed with exit code 101

[ruben-arts]

cc: @pavelzw