Bug: `gen_rust_project` fails to parse `aquery` output when no Rust targets defined

The `gen_rust_project` tool fails with a terse
```
Error: missing field `artifacts` at line 2 column 1
```
when run with a correct `rules_rust` WORKSPACE but no actual Rust targets have been defined.

This is because the returned JSON from `bazel aquery` is an empty dict in this case.