fix: handle null content in SSE stream chunks for REST and Zhipu AI

[octo-patch]

Fixes #1808
Fixes #1788

Problem

When using local LLMs (e.g. llama.cpp) with the custom REST AI endpoint, or when using Zhipu AI, the SSE stream's first chunk typically contains only the role field with "content": null. The two affected listeners passed this null value directly to Message.setContent(), which throws a NullPointerException (caused by a @NonNull enforcement in Lombok-generated code), making the entire AI chat session fail immediately.

Affected files:

• RestAIEventSourceListener.java — used by the custom REST AI integration (issue Bug: #1808)
• ZhipuChatAIEventSourceListener.java — used by the Zhipu AI integration (issue Bug: zhipu api response can't be recorgnized #1788)

Solution

Added a null guard after extracting text from the delta: if text is null, it is replaced with an empty string before calling message.setContent(text). This matches the existing behaviour of FastChatAIEventSourceListener, which already handles this case correctly.

// Before (broken)
String text = ... getDelta().getContent();  // may be null
message.setContent(text);                   // NPE if null

// After (fixed)
String text = ... getDelta().getContent();
if (text == null) {
    text = "";
}
message.setContent(text);  // safe

Testing

• Manually verified the diff aligns with the pattern already used in FastChatAIEventSourceListener.
• The fix is minimal and confined to the two affected listener classes.