OpenAI: Ignore chunks with an empty choices list (elastic#192951)

## Summary

OpenAI added usage stats to their APIs recently:
https://community.openai.com/t/usage-stats-now-available-when-using-streaming-with-the-chat-completions-api-or-completions-api/738156

However, it's somewhat non BWC:

> Note that this usage-specific chunk will have choices: [], so if you
turn this feature on, you may need to update any code that accesses

(Héhéhé)

Leveraging this feature is for another day. This PR is just about
avoiding the whole thing to explode.

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

@@ -90,7 +90,9 @@ export async function getTokenCountFromOpenAIStream({
           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(

x-pack/plugins/inference/server/chat_complete/adapters/openai/openai_adapter.ts

@@ -74,7 +74,7 @@ export const openAIAdapter: InferenceConnectorAdapter = {
       }),
       filter(
         (line): line is OpenAI.ChatCompletionChunk =>
-          '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;

x-pack/plugins/observability_solution/observability_ai_assistant/server/service/client/adapters/process_openai_stream.ts

@@ -98,7 +98,7 @@ export function processOpenAiStream({
         }),
         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;