fix: handle empty JSON responses in sdk client

[seekskyworld]

Issue link:

• Fixes #8014
• https://github.com/anomalyco/opencode/issues/8014

Summary:

• Handle empty JSON responses by falling back to {} instead of throwing.
• Apply the fix to v1/v2 generated clients and keep it via the SDK build patch.

Motivation:

• Some endpoints can return empty bodies, which currently triggers Unexpected end of JSON input in the desktop app.

Validation:

• bun run typecheck (packages/sdk/js)