OpenAI: Ignore chunks with an empty choices list (bis) (elastic#192961

)

## Summary

Follow-up of elastic#192951 to address
the missing bits (given I don't know how to grep)

(cherry picked from commit 08b682f)

x-pack/plugins/actions/server/lib/get_token_count_from_invoke_stream.ts

@@ -288,7 +288,9 @@ const parseOpenAIResponse = (responseBody: string) =>
           delta: { content?: string; function_call?: { name?: string; arguments: string } };
         }>;
       } => {
-        return 'object' in line && line.object === 'chat.completion.chunk';
+        return (
+          'object' in line && line.object === 'chat.completion.chunk' && line.choices.length > 0
+        );
       }
     )
     .reduce((prev, line) => {

x-pack/plugins/elastic_assistant/server/lib/parse_stream.ts

@@ -85,7 +85,9 @@ const parseOpenAIResponse = (responseBody: string) =>
           delta: { content?: string; function_call?: { name?: string; arguments: string } };
         }>;
       } => {
-        return 'object' in line && line.object === 'chat.completion.chunk';
+        return (
+          'object' in line && line.object === 'chat.completion.chunk' && line.choices.length > 0
+        );
       }
     )
     .reduce((prev, line) => {

x-pack/plugins/observability_solution/observability_ai_assistant/common/utils/process_openai_stream.ts

@@ -42,7 +42,7 @@ export function processOpenAiStream(logger: Logger) {
       }),
       filter(
         (line): line is CreateChatCompletionResponseChunk =>
-          'object' in line && line.object === 'chat.completion.chunk'
+          'object' in line && line.object === 'chat.completion.chunk' && line.choices.length > 0
       ),
       map((chunk): ChatCompletionChunkEvent => {
         const delta = chunk.choices[0].delta;