feat(spin_sdk/http): handle empty body in copy fn

Signed-off-by: Vaughn Dice <vdice@akamai.com>

Author: vdice

src/spin_sdk/http/__init__.py

@@ -207,7 +207,8 @@ async def send(request: Request) -> Response:
 
 async def copy(bytes:bytes, tx:ByteStreamWriter):
     with tx:
-        await tx.write_all(bytes)
+        if bytes is not None:
+            await tx.write_all(bytes)
 
 def _trailers_future() -> FutureReader[Result[Optional[Fields], ErrorCode]]:
     return wit.result_option_wasi_http_types_0_3_0_rc_2026_03_15_fields_wasi_http_types_0_3_0_rc_2026_03_15_error_code_future(lambda: Ok(None))[1]