fix: correctly handle merges when 'current' is null

Signed-off-by: Gareth Evans <gareth@bryncynfelin.co.uk>

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());