refactor: handle null RenderResult responses gracefully (#81895)
### What?
- Removed unused `toUnchunkedBuffer` method from RenderResult
- Changed null response handling to return empty values instead of
throwing errors
### Why?
The codebase now provides `RenderResult.EMPTY` for null responses,
making it a valid state rather than an error condition. This change
ensures consistent behavior across the rendering pipeline when dealing
with empty responses.
### How?
- Modified `toUnchunkedString()` to return empty string for null
responses
- Updated `coerce()` to return empty array for null responses
- Fixed `chainStreams()` to return an empty stream when no streams are
provided
- Removed dead code that was no longer used
This refactor improves error handling and simplifies the stream
processing logic by treating empty/null responses as valid states.