ENG-4325: Guard nil rack.input for Rack 3 compatibility

[remmons]

ENG-4325

Purpose

Make the gem compatible with Rack 3. Under Rack 3, env["rack.input"] is no longer guaranteed to be present — it is nil for bodyless requests (e.g. GET), whereas Rack 2 always supplied an empty StringIO. The doc generators call .rewind on it unconditionally, so every GET example raised NoMethodError: undefined method 'rewind' for nil and failed to document.

This blocks the Rack 2 → 3 upgrade in promote (ENG-4325) — all 13 */doc_spec.rb API-doc specs there fail without it.

Approach

Treat a nil rack.input as an empty body in both readers:

• ClientBase#read_request_body — return "" if input.nil? before rewind/read.
• TestServer#call — guard the rewind/read of env["rack.input"] (same nil case).

No behavior change on Rack 2 (input is always present there); on Rack 3 a bodyless request now documents an empty body instead of crashing.

Testing

Verified against promote's Rack 3.1.21 branch with this fork pointed in via a local path: override: all 13 doc_spec.rb examples pass (were 13/13 failing with rewind for nil).