fix: make sure choices is not nil or None to make it compatible with OpenAI

[hustxiayang]

Description
Currently, it sets omitempty for choices, it would cause choices to be None, which is not compatible with OpenAI and caused programs to crash.
see openai-go: https://github.com/openai/openai-go/blob/6723d0b464f1d6898d7242efad6b94cc3ad7dfe6/chatcompletion.go#L183-L184

Fix: remove the omitempty

[dosubot]

Related Documentation

1 document(s) may need updating based on files changed in this PR:

Envoy's Space

connect-providers /ai-gateway/blob/main/site/docs/capabilities/llm-integrations/connect-providers.md

View Suggested Changes

@@ -304,8 +304,18 @@
 4. **Authentication**: The AIServiceBackend's security policy provides credentials for upstream authentication
 5. **Schema Transformation**: If needed, the request is transformed from the input schema to the backend's expected schema
 6. **Provider Communication**: The request is forwarded to the actual AI provider with proper authentication
-7. **Response Processing**: The provider's response is transformed back to the unified schema format
+7. **Response Processing**: The provider's response is transformed back to the unified schema format with strict OpenAI API compliance
 8. **Client Response**: The standardized response is returned to the client
+
+### OpenAI API Compatibility
+
+The AI Gateway ensures strict OpenAI API compliance when transforming responses from various providers. This guarantees compatibility with OpenAI client libraries and tools that expect standard OpenAI response structures:
+
+- **Response Schema Compliance**: All response fields match OpenAI's schema exactly, regardless of the backend provider
+- **Streaming Response Format**: In streaming chat completion responses, the `choices` field is always present as an array, even when empty (never null or omitted)
+- **Consistent Structure**: Streaming chunks follow OpenAI's format precisely, ensuring client applications can reliably parse responses without encountering unexpected null values
+
+This strict compliance prevents issues where client programs might crash when encountering non-standard response formats from different providers.
 
 ## Common Configuration Patterns
 

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.33%. Comparing base (0ef2687) to head (f609f0e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1975   +/-   ##
=======================================
  Coverage   84.33%   84.33%           
=======================================
  Files         130      130           
  Lines       17986    17987    +1     
=======================================
+ Hits        15169    15170    +1     
  Misses       1873     1873           
  Partials      944      944           

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[hustxiayang]

/retest

[hustxiayang]

/retest