[Security Assistant] Fixes LangSmith support for specifying configuration via env vars (#180426)

## Summary

With https://github.com/elastic/kibana/pull/180227, LangSmith
configuration (Project & API Key) could no longer be specified using
environment variables when working locally. This fixes that issue, which
was caused by sending `''` for `langSmithProject` and `langSmithApiKey`
instead of `undefined`.

To test, set the below env vars, then start kibana. Be sure to not have
the UI trace options set as shown in
https://github.com/elastic/kibana/pull/180227.


```
# LangChain LangSmith
export LANGCHAIN_TRACING_V2=true
export LANGCHAIN_ENDPOINT="https://api.smith.langchain.com"
export LANGCHAIN_API_KEY="🫣"
export LANGCHAIN_PROJECT="Best Project Ever"
```

x-pack/packages/kbn-elastic-assistant/impl/assistant/api/index.tsx

@@ -80,8 +80,10 @@ export const fetchConnectorExecuteAction = async ({
     replacements,
     isEnabledKnowledgeBase,
     isEnabledRAGAlerts,
-    langSmithProject: traceOptions?.langSmithProject,
-    langSmithApiKey: traceOptions?.langSmithApiKey,
+    langSmithProject:
+      traceOptions?.langSmithProject === '' ? undefined : traceOptions?.langSmithProject,
+    langSmithApiKey:
+      traceOptions?.langSmithApiKey === '' ? undefined : traceOptions?.langSmithApiKey,
     ...optionalRequestParams,
   };
 

x-pack/plugins/elastic_assistant/server/routes/evaluate/utils.ts

@@ -124,7 +124,7 @@ export const getLangSmithTracer = ({
       return [];
     }
     const lcTracer = new LangChainTracer({
-      projectName: projectName ?? 'default', // Shows as the 'test' run's 'name' in langsmith ui
+      projectName, // Shows as the 'test' run's 'name' in langsmith ui
       exampleId,
       client: new Client({ apiKey }),
     });