Rollup merge of #160063 - xmakro:fix/parallel-dump-dep-graph-ice, r=petrochenkov

Fix ICE when dumping the dep graph with the parallel frontend

`-Zquery-dep-graph -Zdump-dep-graph` ICEs whenever `-Zthreads` is greater than 1:

```console
$ echo 'fn main() {}' > foo.rs
$ rustc -C incremental=incr -Zquery-dep-graph -Zdump-dep-graph -Zthreads=2 foo.rs
thread 'rustc' panicked at compiler/rustc_middle/src/dep_graph/retained.rs:40:63:
called `Option::unwrap()` on a `None` value
```

Reproduces on nightly on 15 of 15 runs at `-Zthreads` 2, 4 and 8. `-Zthreads=1` passes.

Dep-graph indices are handed to threads in batches of 256 and `EncoderState::finish` abandons each worker's unused tail, so `LinkedGraph::add_node_with_idx` back-fills the skipped slots with `data: None`. `RetainedDepGraph::nodes` unwrapped every slot, instead we skip the empty ones.