## Issue
Closes#6122
## Change
`toOpenAiMessage` has two branches for an `AiMessage`, and only the
second one tolerates null text:
```java
if (!aiMessage.hasToolExecutionRequests()) {
.content(aiMessage.text()) // threw on null
}
...
.content(aiMessage.text() != null ? aiMessage.text() : "") // already guarded
```
`ChatCompletionAssistantMessageParam.Builder.content(String)` is
non-null in the SDK, so a message with no
text and no tool calls threw `NullPointerException` before the request
was sent. The fix applies the guard the
other branch already uses, so both behave alike.
The message is one this module produces:
`OpenAiOfficialStreamingChatModel` builds its result with
`.text(text.isEmpty() ? null : text)`, so a stream returning neither
text nor tool calls yields an
`AiMessage` with null text and no tool execution requests. Once that is
in the chat memory, the next call in
the conversation fails while mapping it.
The empty string is what the other branch already substitutes for the
same value, so both branches now send
the same thing for the same input. The SDK does allow the field to be
omitted instead, and some other
integrations skip the text when it is null, but they build a list of
content parts where omitting one is
natural; here `content` is a single field, and omitting it in one branch
while the other sends `""` would
just move the inconsistency rather than remove it.
### Tests
- `InternalOpenAiOfficialHelperTest` (existing class, 2 new tests): an
`AiMessage` with no text and no tool
calls now maps to empty content instead of throwing; an `AiMessage` that
does carry text still maps to that
text
The first fails on unmodified `main` with the `NullPointerException`
above. The second passes either way, it
pins the behaviour that must not change.
```
mvn -pl langchain4j-open-ai-official test
Tests run: 20, Failures: 0, Errors: 0, Skipped: 0
mvn -pl langchain4j-open-ai-official verify -DskipITs
revapi: API checks completed without failures
langchain4j-core: Tests run: 1266, Failures: 0, Errors: 0, Skipped: 5
langchain4j: Tests run: 1358, Failures: 0, Errors: 0, Skipped: 0
```
OpenAI integration tests need an API key and were not run.
## General checklist
- [X] There are no breaking changes (API, behaviour)
- [X] I have added unit and/or integration tests for my change
- [X] The tests cover both positive and negative cases
- [X] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [X] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)
Subhash Polisetti
b8b809ed57