fix(openapi-fetch): handle empty error responses when Content-Length header is stripped

[kaigritun]

Summary

Fixes #2574

When a proxy (like Cloudflare) strips the Content-Length header from an error response with no body, the client would return an empty string as the error value. Since empty string is falsy, this causes issues when checking if (error) - the code incorrectly treats it as "no error".

Changes

This fix applies the same safe handling to error responses that already exists for success responses. When Content-Length is absent and the body is empty:

• Use response.text() first to get the raw content
• If the raw content is empty, return undefined for the error value
• This is consistent with the existing handling for status 204 and Content-Length: 0

Testing

Added two new test cases in delete.test.ts:

1. Error response (500) with empty body and no Content-Length header
2. Success response (200) with empty body and no Content-Length header (verifies existing behavior still works)

All 442 tests pass including the new ones.

Reproduction

// Simulate proxy stripping Content-Length header
global.fetch = async () => new Response(null, {
  status: 500,
  headers: {} // No Content-Length header
});

// Before fix: { error: "", response } - empty string is falsy
// After fix: { error: undefined, response } - consistent with 204 handling

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