Fix chat_template crash when assistant message omits the `content` key

## Fix `chat_template` to handle assistant messages without a `content` key

**⚠️ This template will start crashing for every tool-calling user as soon as the next `transformers` release ships.**

The upstream PR https://github.com/huggingface/transformers/pull/45422 normalizes message inputs by stripping `content=None` before rendering (`None` and absent are semantically identical, and `content=None` is exactly what the OpenAI API returns for tool-call-only messages). That normalization is correct, but it exposes a latent bug in this template: the `tool_calls` branch reads `message['content']` directly, which raises when the key is absent.

Concretely, this code path is hit by **any tool-calling pipeline** (OpenAI-compatible servers, agent frameworks, function-calling demos) that produces assistant messages with `tool_calls` and no textual content. Today most of them happen to pass `content=None` explicitly and get away with it. After the transformers release, all of them break.

### Repro

**Today** (works):

```python
from transformers import AutoTokenizer

tok = AutoTokenizer.from_pretrained("RedHatAI/DeepSeek-R1-quantized.w4a16")
tok.apply_chat_template(
[
{"role": "user", "content": "What's the weather in Paris?"},
{"role": "assistant", "content": None, "tool_calls": [{
"type": "function",
"function": {"name": "get_weather", "arguments": '{"city":"Paris"}'},
}]},
],
tokenize=False,
)
# renders correctly
```

**After https://github.com/huggingface/transformers/pull/45422** (same call, same input — `transformers` strips `content=None` before rendering, so the template sees an absent key and crashes):

```
UndefinedError: 'dict object' has no attribute 'content'
```

You can reproduce the post-release behavior today by simply omitting the `content` key.

### The fix

A one-character change: `message['content'] is none` → `message.get('content') is none`. `.get()` returns `None` whether the key is absent or set to `None`, so both cases are handled identically.

Verified against a 14-case regression suite (single-turn, multi-turn, tool flows with/without final answers, multi-system, `</think>` reasoning, unicode, empty content): all cases either render bit-identically to the current template or, for the previously crashing case, render correctly. **Zero regressions.**

---

*Disclaimer: this PR was opened as part of a scan for repos whose `chat_template` is derived from (or copies) the DeepSeek-R1 template, identified by the presence of the buggy substring `message['content'] is none`. The same one-line fix is proposed wherever that pattern appears verbatim. I do not maintain this model, please review before merging.*

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