fix: correctly handle merges when 'current' is null (#4153)

Fixes #4153

Auto-cherry-pick to 1.0.x

Signed-off-by: Gareth Evans <[email protected]>

Author: garethjevans

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/api/OpenAiStreamFunctionCallingHelper.java

@@ -57,6 +57,10 @@ public ChatCompletionChunk merge(ChatCompletionChunk previous, ChatCompletionChu
 			return current;
 		}
 
+		if (current == null) {
+			return previous;
+		}
+
 		String id = (current.id() != null ? current.id() : previous.id());
 		Long created = (current.created() != null ? current.created() : previous.created());
 		String model = (current.model() != null ? current.model() : previous.model());
@@ -79,6 +83,10 @@ private ChunkChoice merge(ChunkChoice previous, ChunkChoice current) {
 			return current;
 		}
 
+		if (current == null) {
+			return previous;
+		}
+
 		ChatCompletionFinishReason finishReason = (current.finishReason() != null ? current.finishReason()
 				: previous.finishReason());
 		Integer index = (current.index() != null ? current.index() : previous.index());