…ude content field (agentscope-ai#1283)

## Description

When Qwen3 and similar models operate in thinking mode, assistant
messages may contain `reasoning_content` + `tool_calls` but `null`
content. Due to `@JsonInclude(NON_NULL)` on `OpenAIMessage` and
`DashScopeMessage`, the `null` content field was entirely omitted from
the JSON request body, causing strict OpenAI-compatible APIs (e.g. vLLM)
to reject the request with:

```
Input error. Field required: input.messages.N.content
```

### Root Cause

In both `OpenAIMessageConverter` and `DashScopeMessageConverter`, when
an assistant message has `tool_calls` but no `TextBlock`, the `content`
field was left as `null` on the DTO. Jackson's `@JsonInclude(NON_NULL)`
then omitted the field entirely during serialization.

### Fix

Explicitly set `content` to an empty string `""` (instead of leaving it
`null`) when an assistant message has `tool_calls` but no text content.
This ensures the `content` field is always present in the serialized
JSON.

**Changed files:**
- `OpenAIMessageConverter.convertAssistantMessage()` — set `content=""`
when `hasTextContent` is false and tool calls exist
- `DashScopeMessageConverter.convertToSimpleContent()` — replace `null`
content with `""` for assistant messages with tool calls

Close agentscope-ai#1268

## Checklist

- [x] Code has been formatted with `mvn spotless:apply`
- [x] All tests are passing (`mvn test`)
- [x] Javadoc comments are complete and follow project conventions
- [x] Related documentation has been updated (e.g. links, examples,
etc.)
- [x] Code is ready for review

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