Rust: How to handle models with `precompiled_charsmap = null`

Hi guys,
I'm currently working on https://github.com/supabase/edge-runtime/pull/368 that pretends to add a rust implementation of `pipeline()`.

While I was coding the `translation` task I figured out that I can't load the `Tokenizer` instance for [Xenova/opus-mt-en-fr](https://huggingface.co/Xenova/opus-mt-en-fr) `onnx` model and their other `opus-mt-*` variants.

I got the following:

```rust
let tokenizer_path = Path::new("opus-mt-en-fr/tokenizer.json");
let tokenizer = Tokenizer::from_file(tokenizer_path).unwrap();
```

```
thread 'main' panicked at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/normalizers/mod.rs:143:26:
Precompiled: Error("invalid type: null, expected a borrowed string", line: 1, column: 28)
stack backtrace:
0: rust_begin_unwind
at /rustc/80eb5a8e910e5185d47cdefe3732d839c78a5e7e/library/std/src/panicking.rs:662:5
1: core::panicking::panic_fmt
at /rustc/80eb5a8e910e5185d47cdefe3732d839c78a5e7e/library/core/src/panicking.rs:74:14
2: core::result::unwrap_failed
at /rustc/80eb5a8e910e5185d47cdefe3732d839c78a5e7e/library/core/src/result.rs:1679:5
3: core::result::Result::expect
at /rustc/80eb5a8e910e5185d47cdefe3732d839c78a5e7e/library/core/src/result.rs:1059:23
4: ::deserialize
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/normalizers/mod.rs:139:25
5: as serde::de::Visitor>::visit_some
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.207/src/de/impls.rs:916:9
6: <&mut serde_json::de::Deserializer as serde::de::Deserializer>::deserialize_option
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:1672:18
7: serde::de::impls::>::deserialize
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.207/src/de/impls.rs:935:9
8: as serde::de::DeserializeSeed>::deserialize
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.207/src/de/mod.rs:801:9
9: as serde::de::MapAccess>::next_value_seed
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:2008:9
10: serde::de::MapAccess::next_value
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.207/src/de/mod.rs:1874:9
11: as serde::de::Visitor>::visit_map
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/tokenizer/serialization.rs:132:55
12: <&mut serde_json::de::Deserializer as serde::de::Deserializer>::deserialize_struct
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:1840:31
13: tokenizers::tokenizer::serialization::>::deserialize
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/tokenizer/serialization.rs:62:9
14: ::deserialize::__Visitor as serde::de::Visitor>::visit_newtype_struct
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/tokenizer/mod.rs:408:21
15: <&mut serde_json::de::Deserializer as serde::de::Deserializer>::deserialize_newtype_struct
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:1723:9
16: tokenizers::tokenizer::_::::deserialize
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/tokenizer/mod.rs:408:21
17: serde_json::de::from_trait
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:2478:22
18: serde_json::de::from_str
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.124/src/de.rs:2679:5
19: tokenizers::tokenizer::Tokenizer::from_file
at /home/kalleby/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokenizers-0.20.0/src/tokenizer/mod.rs:439:25
20: transformers_rs::pipeline::tasks::seq_to_seq::seq_to_seq
at ./src/pipeline/tasks/seq_to_seq.rs:51:21
21: app::main
at ./examples/app/src/main.rs:78:5
22: core::ops::function::FnOnce::call_once
at /rustc/80eb5a8e910e5185d47cdefe3732d839c78a5e7e/library/core/src/ops/function.rs:250:5
```

I now that it occurs because their `tokenizer.json` file was the following:

_opus-mt-en-fr:_
```json
"normalizer": {
"type": "Precompiled",
"precompiled_charsmap": null
}
```
While the expected behavior must be something like this:

_nllb-200-distilled-600M:_
```json
"normalizer": {
"type": "Sequence",
"normalizers": [
{
"type": "Precompiled",
"precompiled_charsmap": "ALQCAACEAAA..."
}
]
}
```

Looking in the original version of [Helsinki-NLP/opus-mt-en-fr](https://huggingface.co/Helsinki-NLP/opus-mt-en-fr) I notice that is no `tokenizer.json` file for it.

I would like to know if is the `precompiled_charsmap` necessary expect a non-null?
> Maybe it could be handle as `Option<_>`?

Is there some workaround to execute theses models without change the internal model files?
How can I handle an exported `onnx` model that doesn't have the `tokenizer.json` file?