docs: note that req.body may be undefined in v2

[Hashim1999164]

Summary

• Document the v1 to v2 behavior change where req.body stays undefined when no body is parsed, instead of always being initialized to {}.
• Call out optional chaining / defaults for code that assumed an object.

Fixes#632

Test plan

• Confirm the new note reads clearly in the README API section
• Confirm it matches the breaking change already listed in HISTORY.md for 2.0.0

[kilisamemarisaaa]

The "empty body" example is broader than v2's actual behavior. A request with a matching Content-Type: application/json and Content-Length: 0 is still considered to have a body, and the JSON parser returns {}; the existing test/json.js cases "should handle Content-Length: 0" and "should handle empty message-body" both assert that result.

The breaking change applies when no body is parsed, for example when the request has neither Content-Length nor Transfer-Encoding, or when Content-Type does not match. Could this example be narrowed so it does not imply that all zero-length JSON requests now leave req.body undefined?