Don't access pad_token_id if there is no pad_token
When using the `encode`  method of a fast tokenizer, we end up here and accessing `pad_token_id` may log an error even when no padding is done. This PR corrects that.

This fixes #4764 .

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=h1) Report
> Merging [#4773](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cd4e07a85e6161111016ca6d811d97e59368971a&el=desc) will **increase** coverage by `0.34%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4773/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4773      +/-   ##
==========================================
+ Coverage   77.09%   77.43%   +0.34%     
==========================================
  Files         128      128              
  Lines       21059    21059              
==========================================
+ Hits        16235    16308      +73     
+ Misses       4824     4751      -73     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.91% <ø> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.15% <0.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.27% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.03% <0.00%> (+1.35%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `35.03% <0.00%> (+6.36%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4773/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.39% <0.00%> (+14.55%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=footer). Last update [cd4e07a...734e4af](https://codecov.io/gh/huggingface/transformers/pull/4773?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM, also pinging @mfuntowicz for interface of Rust tokenizer and transformers tokenizer